use URL hash fragment to pin UI to a particular state

[jhump]

Currently, refreshing the grpcui page -- for example, to re-query for the remote service definition if the server is restarted with changes to its protocol -- resets everything. It changes tab to the request form, which gets blanked out. The service and method name drop-downs revert to whatever item is first in the list, instead of remembering what was previously selected.

It would be much nicer if the UI recorded its state in a hash fragment in the URL and could then reconstitute the UI state from that fragment when the page is loaded. That way, refreshing does not lose any of the data already entered.