Sync instead of authenticate & always cache locally

[icidasset]

Closes #295 & #342

To do:

• [x] Implement syncing algorithm
• [x] Replace auth screen with intro in app + sync menu
• [x] Replace import/export page with new "data + sync" page
• [x] Remove all things authentication
• [x] Adjust every user storage method to new way of doing things (taskports, etc)
• [x] Backwards compatibility (rename database columns before Elm app starts)
• [x] Thoroughly test syncing algorithm
• [x] New UI
• [x] Only start processing sources after syncing has finished
• [x] Split up "data backup & sync" into two pages: "Choose storage service" and "Manual backup"

[Netherquark]

awesome!