Add ability to delete multiple reminders at once & display mentions in `!remind list`.

[TizzySaurus]

Part 1 of #2231.

Delete Multiple Reminders This has been tested with the help of @ichard26 (thanks!).

The behaviour behind the deletion is still the exact same (you can't delete others' reminders unless you're an admin), just has been reformatted to account for the fact that a lock needs to be applied to each individual id.

There's also some changes to prevent a message being sent for each deletion fail -- we instead send one message after processing all ids.

All reminders successfully deleted:

Some reminders successfully deleted:

No reminders were successfully deleted:

Display Mentions !remind list will now display the .mention attribute, instead of the .name attribute to be clearer.

Before:

After: