Remove FIELD_KEYWORDS, index project slugs as body content

Summary:
D17384 added a "keywords" field but only partially implemented it.

• Remove this field.
• Index project slugs as part of the document body instead.

Test Plan:

• Ran bin/search index PHID-PROJ-... --force.
• Found project by searching for a unique slug.

Reviewers: chad, 20after4

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17596