Use new mw-ui-constructive Agora styles

The new Agora ("beveled bottom") version deprecates mw-ui-primary.
* Use mw-ui-constructive (green) for Login, Create account, and Reset
password.
* Use mw-ui-progressive (blue) for Join <wiki> and Search.

A separate change I90954ea will implement right-aligned and quieter button layout.

Bug: 60596
Change-Id: Ia1fe557e7ebeac011c5da7297e848c1bfee7910f
Merge "Use new mw-ui-constructive Agora styles"