Server: Frontend validation for Add User form

[pixelipo]

New User form badly needs some frontend validation to match the PHP stuff. While pull request #11581 I had to type in the entire form several times because it would empty it up after failing to comply with password requirements:

1. "password too short" - that error has frontend validation, but it's only displayed as a popover on hover, so it's hard to discover
2. "password too common" - resets the form:
   
3. "user already exists" - same as :arrow_up:
   needs uppercase, lowercase, the list goes on.

What should happen:

1. errors should show where they occur
2. location of the error should be highlighted
3. form should not be cleared
   

@nextcloud/designers sorry for my bad "screenshot", but what do you think? Should notifications stay on top (with an "x") to keep the consistency with the rest of the NC?

[nextcloud-bot]

GitMate.io thinks possibly related issues are https://github.com/nextcloud/server/issues/8098 (Adding new user fails), https://github.com/nextcloud/server/issues/5114 (NC12, Adding user not possible), https://github.com/nextcloud/server/issues/4634 (Add ability for admin to hide folders from users ), https://github.com/nextcloud/server/pull/9962 (Added icons to users list and improved design), and https://github.com/nextcloud/server/pull/11417 (Add back the total used space per user).

[MorrisJobke]

New User form badly needs some frontend validation to match the PHP stuff.

And also inline the errors from the backend, because the notification at the top is something we should not use anymore designwise.

[jancborchardt]

The errors like "Password too short" should be shown using the tooltip() function so the interface is not moved around and it calls enough attention to itself.

(Also, we never ever use italic text in an interface. :)

[pixelipo]

That sounds good @jancborchardt - important thing is that these are actually visible for n seconds

[jancborchardt]

Or until the next interaction? (Or until it’s long enough?) Purely time-based messages are always a pain when you don’t look, or they are gone too quick – which is why we are trying to phase out the top showNotification().