OpenAPI compliance: avoid $ref siblings (#1457)

This strives to fix all remaining cases where additional attributes
(most often 'description' but not only) are provided next to $ref
by wrapping $ref in allOf; and also drops allOf in a couple of places
where $ref is the only element under it.

data/api/identity/v2_associations.yaml

@@ -191,7 +191,8 @@ paths:
                   The signatures of the verifying identity servers which show that the
                   association should be trusted, if you trust the verifying identity
                   services.
-                $ref: "../../schemas/server-signatures.yaml"
+                allOf:
+                  - $ref: "../../schemas/server-signatures.yaml"
             required:
               - address
               - medium

data/api/identity/v2_invitation_signing.yaml

@@ -74,7 +74,8 @@ paths:
               signatures:
                 type: object
                 description: The signature of the mxid, sender, and token.
-                $ref: "../../schemas/server-signatures.yaml"
+                allOf:
+                  - $ref: "../../schemas/server-signatures.yaml"
               token:
                 type: string
                 description: The token for the invitation.