Room settings: need a warning when setting a room public without address

[AmandineLP]

We really need a way to warn the user who creates a room and unchecks "Make this room pivate" that it won't be visible until he sets an address.
eg a line appearing in the settings under the box he checked saying "This room has no address so it won't be visible in the directory, even if not private".

Even I get confused

[AmandineLP]

#1059

[dbkr]

I think this is fixed by matrix-org/synapse#664 ?

[ara4n]

i also added the warning.