UX improvements to manage-files and manage-file-sources

This is a follow-on meta issue for files & file sources in a notebook.

- [ ] the "add file" and "add file source" buttons should trigger some kind of "waiting" spinner. If there is a failure, a dialog should appear describing the error. If there is a success, a snackbar should appear. Doing so will allow us to remove the extra list items in the `manage-files` flow, and remove _all_ local business state that exists there now. Paired with #2200, this will bring `manage-files` into harmony with `manage-file-sources`.
- [ ] `manage-file-sources` needs a slight UX refactor for long file titles. In small, tiled situations (eg engineers with a 900px browser tiled next to their editor), overflow issues are likely.
- [ ] when a new file appears but the `manage-files` tab is not selected, there should be a _counter_ next to the `manage files` tab to suggest that there are new items in that view.
- [ ] *stretch goal:* There are _zero_ calls to action after you've added a file / file source. We need to have a simple way for users to now what to do next.