Comment 2 for bug 1458697

Additional thoughts on the HTML move, based on discussion on email, my message there copied here verbatim:

"""
At the summit last week, we agreed to:

1. move base.html over from the framework to the dashboard, and
2. move the _conf.html and _scripts.html over as well, since they configure the application (dashboard).

Upon starting the work it occurs to me that all of the other files referenced by base.html should also move. So, here's the complete list of base.html components and whether they should move over in my opinion:

- horizon/_custom_meta.html
  Yep, is an empty file in horizon, intended as an extension point in dashboard. The empty file (plus an added comment) should move.
  - horizon/_stylesheets.html
  Is just a dummy in horizon anyway, should move.
- horizon/_conf.html
  Yep, should move.
- horizon/client_side/_script_loader.html
  Looks to be a framework component not intended for override, so we should leave it there.
- horizon/_custom_head_js.html
  Yep, is an empty file in horizon, intended as an extension point in dashboard. Move, with a comment added.
- horizon/_header.html
  There is a basic implementation in framework but the real (used) implementation is in dashboard, so should move.
- horizon/_messages.html
  This is a framework component, so I think should stay there. I'm not sure whether anyone would ever wish to override this. Also the bulk of it is probably going to be replaced by the <toast> implementation anyway... hmm...
- horizon/common/_sidebar.html
  This is an overridable component that I think should move.
- horizon/common/_page_header.html
  This is an overridable component that I think should move.
- horizon/_scripts.html
  Yep, should move.
"""