Remove required login for dismissal of in-app notifications

[dennissiemensma]

Someone mentioned me about the implementation of the in-app notifications. Currently an admin login is required to dismiss/delete them, but that doesn't make sense.

The reason for it is that public instances won't allow others to mark these notifications read. But it does seems unneeded, as any admins will check the dashboard after each upgrade anyway.

So I'll:

• Remove the login-required check.
• Convert them to XHR calls.

[dennissiemensma]

Reported by @sdeleeuw

[dennissiemensma]

Done, built XHR as well. Merged to development