Reminders: More robust implementation of mention opt-in button

This solves most, if not all issues from the previous commit.

- A timeout of 5 minutes is enforced - this means the button can no
  longer be used either when the reminder arrives or 5 minutes passes
  since creation, whichever comes first.
- Reminder edits in between creation and button clicks will be handled
  responsibly
  - This includes both edits of duration, mentions, and deleting
    reminders altogether.
- UX is improved. This list of to-be-mentioned users is sent up-front
  with the author included. Instructions to click the button comes right
  after the list.
- No updates to the API or site schema required, as the button message
  will disable itself when it encounters any sort of errors.
- Implementation is also somewhat simplified.

There are probably more improvements, maybe one caveat, but it's like
almost midnight and I want to sleep :/ I sure hope the list above covers
most of it.

Further testing will be done. Now `.remind 10s test` is ingrained in my
muscle memory...

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