Allow theming of changes in project issue table and email

Currently it is not possible for a site administrator to change the way that differences between issues and/or comments are displayed. This is true both for the tables displayed on issue nodes and comments, as well as in email notifications of changes. Besides just allowing a developer to actually theme these displays, this patch does the following:

1. Allows the $changes[]['old'] and $changes[]['new'] arrays returned by project_issue_comment_changes() to provide changes in either string or array form.
   String form:

       'taxonomy_vid_10' => array(
          'label' => 'Vocabulary 10',
          'old' => 'MySQL, pgSQL, javascript',
          'new' => 'pgSQL, newbie',
        ),
   

   Array form:

       'taxonomy_vid_10' => array(
          'label' => 'Vocabulary 10',
          'old' => array('MySQL', 'javascript'),
          'new' => array('newbie'),
        ),
   

   Without this patch the second form (array) would cause errors. This patch fixes that and also provides for different theming of the output based on whether 'new' and 'old' are strings or arrays. See below for example output and use cases.

2. Adds a new hook (hook_project_issue_issue_table_alter()) that allows other modules to add additional rows of metadata to the metadata table that is displayed at the top of every issue. The addition of the hook hook_followup_metadata_changes() from #140473: refactor code to compute the diff on an issue generated by a given follow-up made it possible for other modules to add rows to the metadata tables displayed on comments, but it didn't occur to me at the time that we'd also want to allow modules to add rows to the table at the top of the issue that shows all of the current values of the metadata.
3. Fixes a minor bug introduced in #140473: refactor code to compute the diff on an issue generated by a given follow-up that prevented the "Reported by:" and "Updated by:" fields from being added to the metadata information on the top of emails sent in response to issue creation or followups.
4. Adds some additional information in the comment of project_issue_comment_changes() describing hook_followup_metadata_changes().

I have tested this pretty extensively, but there's a lot to keep track of during testing so it's possible that this patch breaks something, so I'd like to get someone else to test it out as well in case I did miss something.

I've again provided a simple testing module that you can download and enable. Please note that this fake_testing_module is not smart and just adds some data via the two relevant hooks. Therefore, it will add the same data to every comment of an issue.

As an example of how data is displayed, I've also attached some screenshots and have email output below.

For a new issue, the metadata information in an email looks like this:

 Project:      Subscribe
 Component:    Code
 Category:     task
 Priority:     normal
 Assigned to:  Anonymous
 Status:       active
 Reported by:  a
 Updated by:   a
 Vocab 8:      myterm1
 Vocab 8a:     myterm1, myterm2
 Vocab 9:      New term
 Vocab 10:     Javascript, MySQL, pgSQL, IE7, Safari, Firefox
 Vocab 11:     Firefox
 Vocab 12:     newtag1, newtag2, newtag3
 Vocab 13:     newtag1, newtag2, newtag3

This is the original issue

For a followup on that issue, here's how the metadata looks:

 Project:      Subscribe
 Component:    Code
 Category:     task
-Priority:     normal
+Priority:     minor
 Assigned to:  Anonymous
 Status:       active
 Reported by:  a
 Updated by:   site2
-Vocab 9:      Old term
+Vocab 9:      New term
-Vocab 10:     Javascript, MySQL, pgSQL, IE6, IE7, Safari
+Vocab 10:     Javascript, MySQL, pgSQL, IE7, Safari, Firefox
 Vocab 11:     -IE6   +Firefox
 Vocab 12:     -oldtag1 , -oldtag2 , -oldtag3   +newtag1 , +newtag2 , +newtag3
 Vocab 13:     +newtag1 , +newtag2 , +newtag3
 Vocab 14:     -oldtag1 , -oldtag2 , -oldtag3

This is the first follow up to the issue.

Again, let me point out that the Vocab metadata comes from fake_tesing_module and does not actually make any sense. It is there primarily to demonstrate how metadata changes returned as strings and arrays print out. In the second example, you can compare Vocab 10 and Vocab 11 with one another. Both represent the same terms; in the first case all terms for the vocab are printed, whereas in the second case only the terms that were added/removed are printed.