Add an "Assemblies" top level menu

[scottcain]

During a conversation with some RGD devs (@jdepons included) and curators, it was pointed out that it is hard to discover that there are other assemblies in JBrowse. They suggested the addition of a top level "Assemblies" menu that lists other assemblies. What to do on selecting one of these assemblies from the menu is open for discussion: it could open the LGV dialog with that assembly already selected (probably the most straight forward option) or go to the last location visited or the beginning of the the first chromosome in the fasta.

[carolinebridge]

Could leverage the existing assemblies manager in desktop to do this -- remove options to delete or edit assemblies for non-privileged users

[cmdcolin]

What to do on selecting one of these assemblies from the menu is open for discussion: it could open the LGV dialog with that assembly already selected (probably the most straight forward option) or go to the last location visited or the beginning of the the first chromosome in the fasta.

this is sort of the crucial thing to try to figure out UI wise. a UI mockup may help.

linear genome views, by and large, are the main use case for the app so making a streamlined way to "drop someone into an lgv with their assembly of interest loaded" may be useful