Return 403 instead of 400 when profile changes are disabled

[barodeur]

With enable_set_displayname: false (or enable_set_avatar_url: false), refusing a profile change returned the right errcode with the wrong status:

PUT /_matrix/client/v3/profile/@alice:example.com/displayname   (displayname already set)
→ 400 {"errcode": "M_FORBIDDEN", "error": "Changing display name is disabled on this server"}

DELETE /_matrix/client/v3/profile/@alice:example.com/displayname
→ 400 {"errcode": "M_FORBIDDEN", "error": "Changing display name is disabled on this server"}

With this fix:

PUT /_matrix/client/v3/profile/@alice:example.com/displayname   (displayname already set)
→ 403 {"errcode": "M_FORBIDDEN", "error": "Changing display name is disabled on this server"}

DELETE /_matrix/client/v3/profile/@alice:example.com/displayname
→ 403 {"errcode": "M_FORBIDDEN", "error": "Changing display name is disabled on this server"}

The spec defines the 403 response of PUT /_matrix/client/v3/profile/{userId}/{keyName} as "The server is unwilling to perform the operation, either due to insufficient permissions or because profile modifications are disabled", while 400 is reserved for malformed input (M_BAD_JSON, M_MISSING_PARAM, …).

Clients seem to rely on errcode field more than the HTTP Status Code, that change seems safe.

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