Use stemming in the MySQL fulltext search engine

Summary:
Ref T6740. When we index a document, also save a copy of the stemmed version.

When querying, search the combined corpus for the terms.

(We may need to tune this a bit later since it's possible for literal, quoted terms to match in the stemmed section, but I think this wil rarely cause issues in practice.)

A downside here is that search sort of breaks if you upgrade into this and don't reindex. I wasn't able to find a way to issue the query that remained compatible with older indexes and didn't have awful performance, so my plan is:

• Put this on secure.
• Rebuild the index.
• If things look good after a couple of days, add a way that we can tell people they need to rebuild the search index with a setup warning.

We might get some reports between now and then, but if this is super awful we should know by the end of the weekend.

Test Plan:
WOW AMAZING

{F2021466}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T6740

Differential Revision: https://secure.phabricator.com/D16947