There were a bunch of authorization and well-formedness checks in
zerver.lib.actions.do_update_message that I moved to
zerver.views.messages.update_message_backend.
Reason: by convention, functions in actions.py complete their actions;
error checking should be done outside the file when possible.
Fixes: #1150.
This is controlled through the admin tab and a new field in the Realms table.
Notes:
* The admin tab setting takes a value in minutes, whereas the backend stores it
in seconds.
* This setting is unused when allow_message_editing is false.
* There is some generosity in how the limit is enforced. For instance, if the
user sees the hovering edit button, we ensure they have at least 5 seconds to
click it, and if the user gets to the message edit form, we ensure they have
at least 10 seconds to make the edit, by relaxing the limit.
* This commit also includes a countdown timer in the message edit form.
Resolves#903.
This is controlled through the admin tab and a new field in the Realms
table. This mirrors the behavior of the old hardcoded setting
feature_flags.disable_message_editing. Partially resolves#903.
The subscribers list is appended to in `peer_add` events with not
regard for preserving the ordering, and ordering isn't really
important here, so it seems best to just sort it in these checks.
