Fix RH #633169 again.
https://bugzilla.redhat.com/show_bug.cgi?id=633169

Two issues:
a) the ATTENDEE property value is supposed to be URI, specifically
   a mailto: URI for email addresses
b) when the calendar has set no user email backend property, the code
   returns an empty string, which had been used as the attendee address.

This had been discovered while working on a downstream bug:
https://bugzilla.redhat.com/show_bug.cgi?id=1515776

It could happen that the snapshot save got interrupted by other UI
code, one of them being composer close, which causes the composer's
dispose() execution, even it has more than one ref_count. The dispose()
frees the content widgets, which results in use-after-free when
the snapshot save continues its execution. The interrupt was possible
also due to several asynchronous calls being involved during the save.

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1538865

Indentation gap had been added in front of every text in the indented block,
which can be wrong in cases where the text is "split" by an anchor, for example
when added by magic-links. There was used wrong indentation gap in the plain
text version too (3 in UI versus 4 spaces in plain text per level).

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1538775

Reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1485982

Bug 791346 - ‘Disposition-Notification-To’ header doesn’t take into account the ‘From Override’ field’s value

This is related to bug 791291, but doesn't fix it, it rather makes sure that:
a) any autosave snapshot is not overwritten with empty body of the message
b) the snapshot is not deleted after "broken" composer close
c) it's offered to recover the last snapshot (if exists) on close of the composer.

This all makes sure that users will not lose all their work, only up
to one minute of time, which is the interval of the autosave.

One message works fine, but multiple messages failed due to changed
format of x-uid-list content.