[Meta] Project issue settings D7 cleanup

This is a meta task for cleaning up the project_issue-related settings in D7. Currently, we have:

1. Subdirectory for storing issue files (this should just be nuked as per field settings will handle this now).
2. Display user signatures on issue followups (I have no idea if anyone is using this).
3. Reply-to address on e-mail notifications (This could conceivably be adapted to use per-node type settings).
4. Auto-followup user (Again, could easily be per-node type).
5. Auto-close days (Could be per-node type pretty easily).
6. Project selection widget (this should be nuked as it's a field widget now).
7. Site-wide help text for new issues (We've already talked about making this into a per-project field using something like dereferenced_list, but I'm not sure if there is an issue for this ) (edit: could also just use the node submission guideline settings).

Some of these settings should just be entirely removed:
#1571394: Remove 'Issue directory' setting (project_directory_issues)
#1571396: Remove 'Site-wide help text for new issues' setting (project_issue_site_help)
#1571398: Port 'Project selection widget' setting (project_issue_autocomplete)

Others need to be reconsidered since we might want to implement the functionality in a totally different way:
#1571400: Port auto-closing functionality to D7
#1571404: Make the 'Reply-to address on e-mail notifications' setting (project_issue_reply_to) per-node-type

Anything that remains as an actual setting should be moved into a per-node-type config setting visible when editing a node type. We'll probably want a new "Project issue settings" vertical tab on the node type edit form (which will also solve #1569524: Add a 'Project settings' tab on node type edit forms). In that vertical tab, if the "Use this content type as an issue" checkbox (or whatever we call it) is checked, use the D7 #states system to reveal all the other issue-specific settings for the node type:
#1597836: Fix project_issue to use the new Project* node-type settings system
#1571402: Make the 'Display user signatures on issue followups' setting (project_issue_show_comment_signatures) per-node-type
Hrm, at this point, only 1 other obvious per-node-type setting for this vertical tab... ;) There might be more as we continue porting, so I'm not too worried. ;)

Original report by mikey_p

The current Project Issue settings page is showing up in the toolbar at the top level with Modules, People, Configuration, etc. I'd like to move this under Configuration, but I'm not sure what category it should show up under. My current understanding is that the categories under the Configuration page are not alterable and provided by core, so I don't think we have alot of options here.

One crazy possibility is that perhaps as much as possible we should move these settings to exist per-node type since we are allowing multiple node types to act as Projects and Issues.