Fix desktop layout bugs and show more incident actions directly

On-call, the incident queue and the account page all had latent CSS bugs
that only show up once the browser is wide enough to hit the desktop
breakpoint (900px+):

- On-call: .days reset its own margin to 0, which canceled the page-wide
  auto-centering on just that element, leaving the day list pinned to the
  left edge while every other card on the page centered normally.
- Queue: the list pane stayed capped at 340-420px even with nothing
  selected, leaving the rest of the screen empty. It now fills the width
  until an incident is picked, then goes back to list+detail.
- Account, admin user and admin team: .btn and .back-link are inline-flex,
  and margin:auto only centers a block box, so the Sign-out button and the
  two admin back-links sat left of their sibling cards instead of matching
  their width. Wrapped each in a block div.

The incident detail action bar also folded Assign, Add note, Resolve and
Clear acknowledgement into a "More" sheet sized for a phone's width.
Desktop has the room, so it now shows them as direct buttons and hides
More instead; Copy incident stays out of the bar since the header already
has its own button for it.

Filed as niklas/terdut-server#31, #32, #33, #34, each with a screenshot.

internal/web/static/js/adminuser.js

@@ -83,7 +83,9 @@ function render() {
 }
 
 function backLink() {
-  return h('a', { class: 'back-link', href: '/admin/users' }, icon('chevronLeft'), h('span', { text: 'Users' }));
+  // Wrapped in a div: .back-link is inline-flex, and only a block-level
+  // element picks up .view-page > *'s margin:auto centering (app.css:316).
+  return h('div', {}, h('a', { class: 'back-link', href: '/admin/users' }, icon('chevronLeft'), h('span', { text: 'Users' })));
 }
 
 // --- identity --------------------------------------------------------------