[SCHEMA] Add account.name column, and "name" attribute to API namespaces.

Summary:
Right now we have this for Gmail and Outlook, and set it from the OAuth
response. But we really need to save this for all accounts, so that we can set
the `From:` header appropriately when sending mail. (We'll add a field to the
login form for generic/eas accounts.) Also expose it in the API's namespace
representation.

Test Plan: Run migration; curl /n/ endpoint.

Reviewers: kav-ya

Reviewed By: kav-ya

Differential Revision: https://review.inboxapp.com/D620