[nextcloud] Overhaul setup UI

Currently when adding a Nextcloud account via the accounts KCM we end up in a situation with three windows. The KCM opens a Nextcloud wizard window which then opens a new window with a web UI to log in. This patchset moves the web UI into the wizard itself, getting rid of the third window.

The default window size of the wizard is adjusted to accommodate the web UI. This makes the other wizard pages look a bit weird. I'd like to address that in a future MR given I was planning on doing some UI work there anyway.

The server URL page is updated to use a FormLayout, display error in an InlineMessage and have the "Next" button in a footer toolbar.

The services page now displays a proper list of services