PR summary enhancements

- 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](https://en.wikipedia.org/wiki/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_