Expose user avatar URL field in the UI

To be able to change a avatar the URL field needs to be exposed. This
way you can't upload any new files but you can use files which are already
on the server.

The URL needs to be a mxc:// one.

src/components/users.js

@@ -221,6 +221,7 @@ export const UserEdit = props => {
           <TextInput source="id" disabled />
           <TextInput source="displayname" />
           <PasswordInput source="password" autoComplete="new-password" />
+          <TextInput source="avatar_url" />
           <BooleanInput source="admin" />
           <BooleanInput
             source="deactivated"

src/synapse/dataProvider.js

@@ -32,6 +32,7 @@ const resourceMap = {
       ...u,
       id: u.name,
       avatar_src: mxcUrlToHttp(u.avatar_url),
+      avatar_url: u.avatar_url,
       is_guest: !!u.is_guest,
       admin: !!u.admin,
       deactivated: !!u.deactivated,