Prevent web queries from running for more than 30 seconds

Summary:
Ref T10849. This enforces a global 30-second per-query time limit for anything not coming from the CLI.

If we run into another issue with MySQL hanging in the future, this should prevent it from being nearly as bad as it was.

Test Plan:

• Set value to 0, verified the UI threw an exception immediately.
• Set value back to 30, browsed around a bunch of pages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10849

Differential Revision: https://secure.phabricator.com/D15799