Add ability for jbrowse-desktop to open a config file

[cmdcolin]

It could be useful to allow jbrowse-desktop users to open a config that is preloaded with tracks, assemblies, plugins, etc.

This was previously called "Open jbrowse web config" as a button in jb2 preliminary PR but was removed

Also similar to "Open data directory" in jb1 jbrowse desktop

Issues:

Make it easy to understand what it does for the user
If opening from a URL, create a local copy of it probably?

[cmdcolin]

Note that I titled this "Open a config file" rather than "Open a jbrowse web config" because it does not have to strictly be related to jbrowse web configs...could be a jbrowse-desktop thing someone else exported perhaps

[cmdcolin]

dup of #2190