Notification watch settings can silently fail to be saved

After review on the pull request, the final fix differs from the one described in the previous comment:

• Failures are handled with then() / catch() instead of fail().
• The two changes of the combined options are applied by an async performChainedChanges(...changes) helper, which awaits each change before sending the next one.
• performChange() now returns a rejected promise for an unsupported option, so it always returns a promise. The guards comparing against the string 'undefined' are removed instead of being turned into truthy checks.

Out of scope, filed separately:

• XWIKI-25264: the user is still not told when saving fails, and the console message prints [object Object] instead of the reason.
• XWIKI-25265: NotificationsSettingsIT#watchAndRename fails locally when run after the other tests of its class. It's unrelated to this fix.