[FEATURE] Alt pop-up implementation

[jfmcquade]

What? Following on from the failed attempt of #2329, it was agreed on a call that we should build a simpler system for handling pop-ups from scratch.

Why? As noted in #2329 and the associated issue, #2195, the current implementation of pop-ups does not adequately meet the demands of our deployments.

How?

• A new pop-up service
• New pop-up action (e.g. pop_up_beta or pop_up_alt)
  • add action handlers for pop_up_beta: open, pop_up_beta: close
  • options for fullscreen; name of template; close_others (or similar); (dismiss on back nav)
    • dismiss on back
    • subscribe to route changes. on back, close and unsubscribe
    • should probably be default to just dismiss on any route changes, and can fine tune later to dismiss on back and expose as parameter

Other details Add any extra details that you think may be useful.