Set supports_pending_changesets for Plastic.

When I added the supports_pending_changesets flag, I neglected to add it for
Plastic. This broke things.

Fixes part of bug 3044.
Allow FileAttachment.file to store longer file paths.

FileField, by default, limits to 100 characters, which was barely enough
as it was. With the new change for prepending a hash, this was
suffocating.

We now set that max length to 512 characters. While that's far smaller
than the max path length of most file systems, it should be enough for
our use, and is far more than what users have had in the past.
Merge branch 'release-1.6.x' into release-1.7.x
Merge branch 'release-1.7.x'
Allow FileAttachment.file to store longer file paths.

FileField, by default, limits to 100 characters, which was barely enough
as it was. With the new change for prepending a hash, this was
suffocating.

We now set that max length to 512 characters. While that's far smaller
than the max path length of most file systems, it should be enough for
our use, and is far more than what users have had in the past.
Merge branch 'release-1.7.x'
Allow commenters to change the status of the issues they open.

Until now, only the owner of a review request could change the status of an
issue. A frequently recurring request is to allow the person who originally
opened the comment to do the same--especially to be able to reopen it.

This is an update of Niklas Hambuechen's change at
https://reviews.reviewboard.org/r/3837/

I've updated it to address the original concerns with that change, as well as
streamlined a lot of the code.

Fixes bugs 2713 and 2888.

Testing done:
- Ran unit tests.
- Verified that the "interactive" parameter in the template was correct.

Reviewed at http://reviews.reviewboard.org/r/4380/