Add command to silence all currently firing alarms in the room

[HarHarLinks]

!silence only stops one alarm. there should be a single command to silence them all (for the current room).

variant: !silence all

• pros: user expectations?
• cons: occupies the word all with a semantic meaning. we have some words with semantic meaning already, like every and cron, but it is possible to still clearly identify their semantic vs label use. all would mean that we need to avoid alarms being created with a label all.

variant: !silenceall

• pros: single command keeps the all free so it can still be used on the reminder text
• cons: ?