Simplified/centralised version dropdowns population (#1500)

[daveoconnor]

This is related to ticket #1500

I did this as two separate commits and it's probably a good idea to evaluate this PR that way:

1. the centralization of the dropdown population to be based around get_dropdown_versions (née version_dropdown/version_dropdown_strict) with args.
2. The movement of that to a higher level so it's set earlier in the flow.

I chose to do point 2 in the BoostVersionMixin so we can use the data in a dispatch() or other similar method if needed instead of being limited to it only being set for the templates, and that potentially causing different datasets being populated or used by unfamiliar devs in the future.

I'm not sure I like this, which is why I set it as two separate commits so it can be evaluated separately more easily. I think it might just be making the code more complicated for not a whole lot of benefit and it's not really saving us much in terms of queries, we need the partial to do one we wouldn't need otherwise because get_object() would otherwise provide that Library object, as you can see by the difference, but we can discuss it on Monday.

I also think we can probably get rid of path("homepage-beta/", HomepageBetaView.as_view(), name="home-beta"), and the related code, that uses a versions dataset but seems like it's not referenced anywhere and hasn't been discussed on the discord channel in over a year.