[Public rooms] - List all the forums from the rooms directory

[giomfo]

We implement here the first step of the option "Rejoindre un forum depuis l’annuaire" of the floating button (+):

• retrieve all the forums (public rooms) from all the known homeservers - see the endpoint publicrooms
  Notes:
• this was done in tchap_v1 by preparing a list of hosts see mCurrentHosts. Then a request was triggered on each host to retrieve its forums see initPublicRooms
• no pagination is used in order to get all the forums (see PUBLIC_ROOMS_LIMIT = -1)
• order all the forums to display first the most populated
• the search is supported
• remove the room creation option from the header (if the screen is the Element one)

Note the related code in tchap-ios is the PublicRoomService