fix(login): Disable login button when already logging in

[SystemKeeper]

Summary

Fixes "State token does not match" in some cases (e.g. double click on the login button)

How to reproduce:

• Start a login flow (V1 or V2), e.g. by logging in via the desktop client
• Enter username and password
• Click "Log in" and when the button changes to "Logging in" click the button again
• You're now prompted with a "temporary error".
• Enter your password
• See the error "State token does not match"

Before

After

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[ChristophWurst]

To prevent double submit through enter we could adjust LoginForm.vue and ignore form submit while loading=true as well. Apparently this would be possible with a disabled field set: https://stackoverflow.com/questions/3507958/how-can-i-make-an-entire-html-form-readonly

[SystemKeeper]

Right, I didn’t think of enter. Let me adjust the PR
Edit: let’s disable the button anyway.

[SystemKeeper]

Apparently this would be possible with a disabled field set

I tried that, but then I am unable to login in any way (it always ends up in a temporary error). Therefore I prevented another submit of the form, as we already have a submit handler.