Improve the Release status dashboard (#547)

Quick branch info (status, RM name, release schedule link)
for all branches at the top, with detailed per-branch sections
after that.
Details can generally be expanded/collapsed.

Show tier-3+ failures: these aren't considered release-blocking
but we tend to fix these ASAP. These mark a branch/builder "yellow".

Show low-priority problems: disconnected builders, build warnings,
unstable builder failures. These keep a branch marked "green".

Show changes and test results for failing builds (when available).

Show build history overview for builders; this is useful context.

Conceptually, the branch sections list "problems", not "builders",
in preparation for adding GitHub blocker issues here.
Problems are sorted by severity & grouped by type; the
highest-severity problem determines the overall branch status.

Based on Pablo's existing dashboard and his BuildbotHammer tool.

Author: encukou