Refactor enrollment and its docs

[algal]

This makes a few changes:

• fixes a broken test, which was not properly failing on attempts to enroll an already enrolled database (a situation which could clobber and decrement a live database's version number)
• refactor enrollment code and docs, so that it does not run migrations (which was a no-op).
• Rewrite the enrollment documentation, to lead with a HOWTO section, and then provide an optional discussion section later.

[algal]

@jph00 This is ready for your review, please.

[jph00]

lgtm @algal :)