Fix for Uncaught Exception and Crash, Lost Messages in Unsynchronized Local-Only Folders

[dnicules]

These are our fixes to address issues #7688 and #823

Uncaught Exception, Crash: Issue #7688
Problem:

• Previously, if a user goes offline, moves a message into another folder, opens the message, and stars/unstars it, the app would consistently crash
  • This did NOT happen when a message was starred/unstarred from the List View.

Solution:

• Now we catch an exception in queueSetFlag() to avoid the crash; this is how it is handled in List View.
  • List View catches the exception in runInBackground()

"Lost" Messages in Unsynchronized, Local-Only Folders: Issue #823
Problem:

• A user using Thunderbird with an account having no archive folder reported that, on moving a message to archive, they were not able to move it elsewhere in any way; the message effectively became local-only and lost to the server.
• Potentially happens with other folders that only exist locally but not on the server

Solution:

• Online Fix: If a user has connectivity, refresh the folder list before making any move operations
• Offline Fix: A dialog/prompt shows up, informing the user that the operation might lead to data loss
  • The user is allowed to proceed anyways, cancel the operation, or to proceed and never show the warning again
• Applies to moving (to arbitrary folders, or also to drafts and spam), copying, archiving.

Authors: @dnicules @gkhourii