Repository dialog design updates

It doesn't feel like the repositories dialog is performing all that well.

• It's hard to read the list
  • It's difficult to scan
  • Hard to identify each item: what type of repo is it? Where did it come from? etc. General problem of ascertaining repo identity.
• Currently we don't show the "main" repo and there's a text at the top to explain this, but that gets a bit lost. It would be more intelligible to show this repo in the list and maybe prevent people from disabling/removing it.
• Smaller UI issues: the lists ought to be rounded, the expander is a bit odd and non-standard.
• Fedora-specific: the 3rd party repos part is due to change (downstream ticket)
• Also in Fedora-land, there's been some discussion about making it possible to configure the priority of repos. I'm a bit uncertain about that, but this would be a good opportunity to look at it in more detail.

There's some other background discussion about this here: Teams/Design/software-mockups#5 (closed) .

I can work on the design for this, but to do so I'll need some assistance from developers. One particular question is around what metadata we have for different types of repo. I'm also keen to ensure that this work aligns with plans for how to handle different repos elsewhere in Software, including how to handle multiple installs of the same app (see #1167 (closed)).

@bertob @jimmac