[SCHEMA] Separate tables for backend folders/labels and Inbox labels.

Following a design discussion, we are splitting our notion of folders
and labels into two different things: folders/labels from the account
backend (e.g. Gmail), and Inbox-specific labels.
Port Gmail code to new folder/label schema.

With this change, we rip out folder name translation for Gmail labels
and store them as-is, lowercased.
[SCHEMA] Save more folder names.

TODO: update  etc. here.