PR summary enhancements

[fcollonval]

• in the PR summary,
  • the chat messages could be a lot more condensed and probably don't need any border at all, nor are extra layout colors needed...
  • the PR source/target repo and key (this could be the link instead of the View Details button,
    • e.g. [octocat]jupyterlab/pull-requests#16)
      • the provider could hoist this SVG instead of cooking it into the typescript build
  • the proposer (also with a link to their page)
  • a tall tree or wide miller columns of the files that are actually in that pr that open the tear-aways, with better metadata than +/-
    • the file browser is just going to need a lot of work, and really needs first-class, first-party support, potentially via that datagrid merged cell PR on lumino (think i mentioned above)
  • the "start a new discussion" box could always be visible, and stuck to the bottom of the tab
    • making it automatically scale to what you're writing would be fly, but a SplitPanel would be fine, too
  • i don't know if the "reply" on a past message makes much sense, unless the provider actually supports them (GH sure doesn't, buh)
  • i'd expect too see the why changing metadata in the summary, with a link to open that diff you are seeing

Originally posted by @bollwyvl in https://github.com/jupyterlab/pull-requests/issues/16#issuecomment-791439670

[fcollonval]

a tall tree or wide miller columns of the files that are actually in that pr that open the tear-aways

Some users gave feedback it is surprising not seeing the modified files when clicking on the PR description panel.