Merge two of the room join codepaths

There's at least one more to merge in.

Side-effects:
 * Stop reporting None as displayname and avatar_url in some cases
 * Joining a room by alias populates guest-ness in join event
 * Remove unspec'd PUT version of /join/<room_id_or_alias> which has not
   been called on matrix.org according to logs
 * Stop recording access_token_id on /join/room_id - currently we don't
   record it on /join/room_alias; I can try to thread it through at some
   point.

synapse/types.py

@@ -13,7 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from synapse.api.errors import SynapseError
+from synapse.api.errors import SynapseError, BadIdentifierError
 
 from collections import namedtuple
 
@@ -51,13 +51,13 @@ class DomainSpecificString(
     def from_string(cls, s):
         """Parse the string given by 's' into a structure object."""
         if len(s) < 1 or s[0] != cls.SIGIL:
-            raise SynapseError(400, "Expected %s string to start with '%s'" % (
+            raise BadIdentifierError(400, "Expected %s string to start with '%s'" % (
                 cls.__name__, cls.SIGIL,
             ))
 
         parts = s[1:].split(':', 1)
         if len(parts) != 2:
-            raise SynapseError(
+            raise BadIdentifierError(
                 400, "Expected %s of the form '%slocalname:domain'" % (
                     cls.__name__, cls.SIGIL,
                 )
@@ -69,6 +69,14 @@ class DomainSpecificString(
         # names on one HS
         return cls(localpart=parts[0], domain=domain)
 
+    @classmethod
+    def is_valid(cls, s):
+        try:
+            cls.from_string(s)
+            return True
+        except:
+            return False
+
     def to_string(self):
         """Return a string encoding the fields of the structure object."""
         return "%s%s:%s" % (self.SIGIL, self.localpart, self.domain)