Prevent crashes when unable to get/create a ReviewRequestVisit.

Under some unknown race condition, it's possible that looking up or
creating a ReviewRequestVisit will fail when viewing a review request.
This isn't the worst thing in the world, and is likely a temporary
problem (probably due to two requests for creating the same
ReviewRequestVisit at once).

Instead of crashing, we now log the failure and continue on.

Reviewed at https://reviews.reviewboard.org/r/5959/
Do a better job of escaping text in commentcounts tag.

If a user set their full name to include </script>, they could cause some
problems. Most of this was fixed in Djblets 0.7.30 and 0.8.3, but the
commentcounts tag in Review Board 1.7.x didn't use json_dumps, and therefore
still had the problem.

This change adds some more escaping for different parts of the commentcounts
structure that gets serialized.

Testing done:
* Tested the view_diff.html template.
* Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/5955/
Add draft release notes for 1.7.26

This adds draft release notes for the bug fixes, security fixes, and one new
feature since 1.7.25.

Testing done: Built HTML docs.

Reviewed at https://reviews.reviewboard.org/r/5956/