Improve login screen

[hurradieweltgehtunter]

Description

When the new OIDC / AzureAD Login was presented the old-fashioned grey "Login with AzureAD" button did not represent this modern feature accordingly. Also the missing dedicated Login-Button was bugging me for years.
This PR adds some styling, makes the HTML valid and adds a dedicated login button.

How Has This Been Tested?

Latest Firefox, Latest Chrome, Latest Safari

Screenshots (if appropriate):

Before:

After:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

QA

• (1) Deleting the background shows weird behaviour
• (2) Setup screen needs some adjustments
• (3) When setting a password in setup screen the password indicator is inside the password field

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[hurradieweltgehtunter]

One thing to keep in mind (mentioned by @ChrisEdS ): For custom brandings the button color should be included in the styling process.

[micbar]

@pmaier1 @hurradieweltgehtunter Ok for me to keep the arrow and add the login button.

@phil-davis but we need to make sure that we really press the login buttton in the CI

[hurradieweltgehtunter]

There's multiple possibilities:

1. Keep it as it is now (keep the arrow) -> no functional changes, tests should pass; Okayish UX
2. Remove arrow & put loading animation in the login button -> tests need to be adapted if they target the arrow submit button in specific; Best UX

I'd prefer 2) UX wise. Let me know if I should do this change also.

[AlexAndBear]

Beautiful 1 !

@hurradieweltgehtunter encountered a little bug with IE11:
clicking in the input field will clear the placeholder

[AlexAndBear]

One more thing: hovering over the -> login arrow button, will now make the item disappear
(Only in Chrome, IE11 works)

[hurradieweltgehtunter]

@ChrisEdS yould you re-check with errors end customer themes?

[hurradieweltgehtunter]

Changes are final, ready to merge.
owncloud/guests#446 needs to be merged too

[ChrisEdS]

@hurradieweltgehtunter Please document how the users/customers can theme the new elements. That should be part of the admin release notes as well. // cc @pmaier1

[AlexAndBear]

@cdamken old version ? invalidate your cache please
@hurradieweltgehtunter I will update the before/after images in the entry description

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
10 Code Smells

No Coverage information
No Duplication information

[phil-davis]

LGTM. There are 50 commits here, and the rebase process was giving me conflicts...

I squashed all the commits into another branch on top of current master - see PR #38821 - we could actually merge the code from that PR with a single clean commit. The changelog link could be left as it is, pointing to this PR, because this PR is where all the discussion and history is.