Handle the forums directory with the Spaces logic

[giomfo]

We want to remove here the use of the endpoint /publicRooms, mainly because it is limited to a unique HS.
In Tchap, we want to list the public rooms (forums) from all the Tchap instances.

We ported the TchapV1 algorithm to keep access to all forums (#117) in the first release of TchapV2. The purpose of this issue is to discuss with the Element Application teams how to replace the existing rooms directory with a public space.

I consider the plan will be:

• create a default public space on each Tchap instance, except for agent.externe. Name it like the instance (Dinum, Interieur...). Add all users of the instance in this space.
  -> we will need for that to have an admin. Can we plan to use the super admin account created on each instance ("Administrateur Agent-xxx...")?
• Add all the existing public rooms (forums) of an instance into its public space
• The power level required to add/remove a room to these spaces will be the minimum (0), to let people add/remove forums by using the Tchap client
• In order to give access to all forums from only one space, define a meta-space on each instance and group all the creates spaces. Name this meta-space "Tchap forums". Add all users of the instance in this space.
• Update the client in order to load the meta space of the user's homeserver when he selects the option "Explore public rooms"