Add a "Follow" button to the issue metadata block

Screenshot attached:

I'll be taking this one on...

... to clarify, I'll be taking this on once the project dev site is updated to where it was a few days ago, since there were quite a few theming changes to the issue pages.

The issue for the other theming that this is postponed on:
#1696872: Create a block that shows the issue metadata

And I think this is what the new buttons are supposed to look like, with the existing issue meta-data block:

I'm going to merge this with #1696872: Create a block that shows the issue metadata and provide a combined patch.

Actually, this is duplicate with these:
#1628042: Implement the "Update this issue" button when viewing an issue
#1560010: [META] Port issue following functionality to D7

Actually, #1560010: [META] Port issue following functionality to D7 resulted in flag_tracker getting built, but that still doesn't render a follow button when viewing an issue. You're supposed to be on your own for placing flag links. So, we might as well use this issue to actually render the flag link in our existing issue metadata block if flag_tracker is enabled and properly configured. I'll be working on this, stay tuned.

This still needs some theme help, since it's just showing up as a standard unthemed flag link. We want to turn it into a big action button. It'd kinda be nice to do that in project_issue (if we're already doing that for the "Update this issue" link), but maybe we should just leave this as bluecheese's problem for now.

Committed #10 to project_issue and pushed to the 7.x-2.x.

Moving this to the bluecheese queue for theme help. Please let me know if you need any markup assistance from me.

Cheers,
-Derek

Note: we want this to look like the follow button (and follower count) on D6, not what's seen in the mockup in comment #6...

needs theming

Actually, Senpai prefers a separate issue for theming:

#1828652: Theme the "Follow" button in the issue metadata block

Moving this back to project_issue and marking fixed.