Add test cases for new Location.visible field

[#85003680]
Add `visible` field to Location model

And filter the `location_list` based on `visible=True`

Default value is `True` so that existing models in the database remain
visible.

https://www.pivotaltracker.com/story/show/85003680 [Delivers #85003680]
Merge pull request #122 from sealevelresearch/add-visible-bool-to-location

Add `visible` field to Location model