Add Server Name format

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

data/api/client-server/content-repo.yaml

@@ -605,6 +605,7 @@ components:
       example: matrix.org
       schema:
         type: string
+        format: mx-server-name
     mediaId:
       in: path
       name: mediaId

data/custom-formats.yaml

@@ -37,6 +37,11 @@ mx-room-id:
   url: /appendices#room-ids
   # regex: "^!"
 
+mx-server-name:
+  title: Server Name
+  url: /appendices#server-name
+  # no regex
+
 uri:
   title: URI
   url: http://tools.ietf.org/html/rfc3986