Closes #3769 Initial save to drafts functionality
Closes #6191 Edit drafts
Closes #7236 Replace previous draft
Merge branch 'master' into feature/drafts
Closes #7253 Ask to save draft
Closes #7237 Delete corresponding draft when sending mail
Closes #7307 Overwrite existing drafts
Ignore case in domain names when warning about link destinations: Closes #7327
Fixes crash when removing unfulfilled during writebehind operation

Encountered this morning -- list operation had unfulfilled emails
waiting for network operations but folder normalization determined
they were missing and used a writebehind to remove them from the
operation, but the loop to remove them explodes (can't do a remove
within a foreach).  This loop solves that problem.
Content menu missing labels in composer: Closes #7322
Build with --enable-ref-tracking: Closes #7262
Background sync'ing should stop when vector is complete: Closes #7325

Account synchronizer now stops when it recognizes that all the mail in
the folder is present (but perhaps not completely downloaded) from
the server.  The email prefetcher takes care of ther rest.

Also discovered a regression from last week's commit that prevented
vector expansion when using find_earliest_email_async(); fixed here
as well.
Reply not included due to bogus References: field: Closes #7333
Merging in some bux fixes from conversation-mop-up branch

Charles discovered that some of the operations in ImapEngine.GenericFolder
were exiting before waiting for the command to complete, meaning the
async call returned while the operation was still running.  This
has a broader problem if the command is executed (with a yield) and
then the folder is closed.
Merge branch 'master' into feature/drafts
Merge branch 'master' into feature/drafts
Autosave drafts. Closes #6124
Don't consider out-of-folder emails when sorting conversation list: Closes #6873
Sort conversations by date received, not sender's date: Closes #6856

Also displays date received in conversation list.
Display oldest unread message as preview: Closes #5921