Partially addresses #47424.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

1f11096c included new filters but made it hard to determine which dropdown did
what. This patch restores the original placeholders.

The percent_complete method returns a value of 100 when a
ZeroDivisionError occurs. That seems like a very strange default for an
error case, and results in a bug when a milestone has no corresponding
issues (new, empty milestones show 100% completion). This commit changes
the rescue value to 0, and subsequently fixes #1656, which reported this
problem.

Show all issues and merge requests that could be considered opened or closed, includes reopened and merged.