Make the topbar configurable with the PyData theme topbar code

[choldgraf]

The PyData theme recently updated its topbar code so that it can be configured with these three options:

...
"navbar_start": [list-of-templates],
"navbar_center": [list-of-templates],
"navbar_end": [list-of-templates],
...

That logic is here: https://github.com/pydata/pydata-sphinx-theme/blob/master/pydata_sphinx_theme/docs-navbar.html

We could populate our topbar using the same template logic, so that we're re-using the configurability mechanism. This could let people configure the topbar in similar ways.

Tasks

• Should we rename navbar to topbar and then translate into the PyData theme config name of "navbar"?
• Swap out our topbar code with the configurable pydata sphinx theme options
• Tests passing