[SCHEMA] Correctly set public_id and exposed_name for existing 'Inbox' folders.

This commit does not actually alter the schema, but does include a migration.
The previous migration was not correctly setting the public_id and exposed_name
fields on existing Gmail 'Inbox' folders. This fixes that.
Put the latest migration where it belongs.
Standardize Crispin types.

We now always coerce inputs to strings before passing them off to the
IMAPClient connection, and convert numbers to long ints before returning
them to the caller, so callers don't have to check types or coerce
themselves.

We are standardizing on *always* treating UIDs, X-GM-MSGID, and
X-GM-THRID as integers when just passing them around.
Don't make type coercions in db wrapper functions.

We should always be returning the straight-up types defined by the db columns.
[DEPENDENCIES] Add clear-cache tool.

Whenever we make an upgrade that e.g. changes the format, data types, or
other parts of cached data, we need to clear the existing cache on a
server restart. Here's a tool to do that.

I chose to use the click library for this tool because it's a really
nice interface for writing CLI scripts. We should use it more going
forward!
More log messages.