enable multiple launchers (with paths)

[bollwyvl]

• fixes Allow mulitple launchers to different paths for a single server #266
• fixes Pass URL filename and URL parameters to browser #222

This reworks the configuration in a backwards-compatible way to enable multiple launchers from the same server process.

The public REST API would change (from launcher_entry to launcher_entries), as does the URL scheme for the icons <server.name>/<launcher.name>.

I opted not to do the submenus, but could.

Additionally, in lab, this makes launcher and layoutrestorer optional, and adds mainmenu and commandpalette, which should make this compatible with jupyterlab-classic (though it wouldn't be exposed in the son-of-tree page, and is probably out of scope for this PR).

I didn't see where launcher_entry is under test, but am happy to add something if requested.

[bollwyvl]

ha, bash ate my commit message, but the most recent push should be:

add docs for `name` and `path`

[bollwyvl]

makes watch work as advertised. the typescript/rimraf stuff is just a ride-along.

[yuvipanda]

Better nate than lever. Sorry for the late response in reviewing this, @bollwyvl. Wanna rebase and we can try land this?