[rbslack] Fix escaping of review request summaries.

We were using Django's escape() for review request summaries, which
covers a variety of characters. However, Slack expects that only &, <
and > will ever be escaped, and will show any other entities as-is.

This change adds our own escaping logic in order to not leak entities in
the summaries.

Reviewed at https://reviews.reviewboard.org/r/6570/