Improvements to login page for better extensions

[Nicklas Nordborg]

The login page can to some extents be modified by extensions but there are some improvements that can be made:

1. The possibility to execute scripts before the login information is submitted. This could for example, be implemented as a custom event that is sent to the form before the form is submitted. Event handlers can do their processing and may also cancel the submission if needed.

2. The possibility to add extra attributes to the login form and/or fields. We already have the DynamicActionsAttributes interface for this and it should be very easy to add support for it.

3. The error message section should always be included in the HTML, but it should be hidden if there is no current message. Extensions should be able to display error messages.

4. It should be possible to store session settings in the SessionControl even if a user is not logged in. This would, for example, be useful for the WebAuthn implementation (​https://baseplugins.thep.lu.se/wiki/net.sf.basedb.webauthn) that need to store information between two requests.

5. More...?