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.
Rishi Gupta