Set load_on_pending for Part.message.

Like commit 526f9620, fixes errors when constructing the transaction log.
Add LOGDIR back to config.

This is actually still needed with the new logging infrastructure, because we
save messsages with decode errors to separate files in that directory in
inbox.models.message._get_errfile_name().
Support read and update operations on user-created tags.

Summary:
Addresses issue #45. Deletes will take a little more care because we need to
ensure that the tag gets removed from all threads.

Test Plan: Unit tests added.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D240
Fix folder mapping KeyError when starting some IMAP syncs.

We were creating the {name: id} mapping for folders before we had
necessarily created folder objects for all folders belonging to an IMAP
account, which meant that sometimes we'd get a KeyError starting up the
sync because we couldn't find a Folder object for a given folder.
[SCHEMA][DO NOT MERGE] Store client_id and client_secret on GmailAccount objects.

This is an interim CL to allow account syncs to run using a developer's client
ID and client secret, as opposed to the ones specified in the sync engine
config. If these parameters aren't stored, we still use the configured ones, so
that auth and sync in the dev VM work as before.

Known issues: breaks contacts sync.
[DO NOT MERGE] Bump db pool size to 200.