Public rooms: turn on the option to join only on invite

[giomfo]

We named "Public rooms" all the rooms listed in the rooms directories.
They are currently joinable without invite from their preview.

DINUM wants to introduce a new access mode for the Public rooms. For some of them, the user will have to ask access to the room admin(s) (= knock mode).

This change requires some designs. Here are the opening questions:

• Shall we limit the room history to the room members? (Prevent others from previewing the room)
• How does the user contact the room admin(s)? by using a direct chat (DM) or a "knock" event?

Note: This request corresponds to the knock mode under discussion in the matrix spec (see MSC2403: Add "knock" feature). Implementation is in progress.