[SCHEMA] Split folders into account backend folders/labels and internal tags.

Summary:
Note that associated fixes for sending mail are in a dependent revision, D140.

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

Following a design discussion, we are splitting our notion of folders
and tags into two different things: folders/labels from the account
backend (e.g. Gmail), and Inbox-specific tags, called InternalTag
(trying to reduce term overloading here).

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. Special Inbox labels will be provided via
different queries; they no longer are stored specially in the database.

Split actions into local/remote, subsuming old API functions.

Since we are now mirroring folders/labels from remote accounts to our
local data store as-is, we no longer have a unified API for what e.g.
'local archive' means for any given message. We solve this by expanding
the actions API to also cover local actions.

Test Plan: ./runtests and ./runtests -i (Kavya's handling send & EAS)

Reviewers: mg, kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D134