Offer "alias" field in room create form

Tries its best to not allow aliases that are too
long (full alias including leading #, : in the
middle, and homeserver domain name must not exceed
255 bytes.

Change-Id: I1e784a94cb599eca7e30736d666b20e37aad5444

src/synapse/dataProvider.js

@@ -83,8 +83,10 @@ const roomCreationProvider = {
     const newParams = { ...params.data,
       public: undefined,
       room_name: undefined,
+      alias: undefined,
 
       name: params.data.room_name,
+      room_alias_name: params.data.alias,
       visibility: params.data.public ? "public" : "private",
     }