Rework routing, data structures and naming ambiguities

[ChristophWurst]

This changes a lot of things

• Mailboxes are identified by their database ID (as opposed to account ID and name)
• Messages are identified by their database ID (as opposed to account ID, mailbox name and message UID)
• Routes are simplified and use the browser history API (as opposed to client-only anchor routes)
• The Vuex store got a lot simpler, because the normalization is obsolete due to the naturally unique IDs
• Naming inconsistency of maibox/folder finally resolved -> we call it "mailbox" in the code (as that is the proper name by the standards) and "folder" for the user (because @jancborchardt prefers that)

Required for deep links in #3433 and possibly for threading.

• (accountId, folderId) to mailbox ID
• Drafts handling (with db IDs)
• Sync handling (with db IDs)