Merge branch 'mjog/draft-errors' into 'mainline'

Misc draft management issues

Closes #858 and #1044

See merge request !613

(cherry picked from commit a21982ae)

6530c66e Geary.Imap.FolderSession: Fix null param critical with vala 0.50
6f1f94e5 Composer.Widget: Suppress unsupported draft folder messages
be793831 Geary.App.DraftManager: Wait for remote to be open in ctor
6d5b0bc5 Composer.Widget: Rework draft manager management
36daf801 Composer.Widget: Clean up set_save_to_override method signature
5533ae32 Composer.Widget: Hide save button by default
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