`!mjolnir rooms` should put the returned room list in a code block

**Is your feature request related to a problem? Please describe.**
When you have lots of protected rooms, the list returned from `!mjolnir rooms` takes up a lot of space and can be cumbersome to scroll through in the timeline.

**Describe the solution you'd like**
Mjolnir should put list list returned from `!mjolnir rooms` into a code block as they are collapsed by default in Element.

**Describe alternatives you've considered**
None comes to mind

**Additional context**
None

Start by locating the handler for the `!mjolnir rooms` command. Check how its returned room list is formatted, then verify in Matrix/Element that the complete list appears in a code block and is collapsed as expected.

Written by the indexing model from the issue text.