Provide a setup warning about using the default MySQL stopword file

Summary:
Fixes T2605.

• Add a setup warning about the stopword file.
• Provide a simpler stopword file.

Test Plan:

• Hit setup warning.
• Resolved it according to instructions.
• Added "various" to a task, then searched for it, found the task.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2605

Differential Revision: https://secure.phabricator.com/D10258