FIX: IMAP sync email update uniqueness across groups and minor improvements

[martin-brennan]

Adds a imap_group_id column to IncomingEmail to deal with an issue where we were trying to update emails in the mailbox, calling IncomingEmail.where(imap_sync: true). However UID and UIDVALIDITY could be the same across accounts. So if group A used IMAP details for Gmail account A, and group B used IMAP details for Gmail account B, and both tried to sync changes to an email with UID of 3 (e.g. changing Labels), one account could affect the other. This even applied to Archiving!

Also in this PR:

• Fix error occurring if we do a uid_fetch and no emails are returned
• Allow for creating labels within the target mailbox (previously we would not do this, only use existing labels)
• Improve consistency for log messages
• Add specs for generic IMAP provider (Gmail specs still to come)
• Add custom archiving support for Gmail
• Only use Message-ID for uniqueness of IncomingEmail if it was generated by us
• Various refactors and improvements