Prior to this commit, we used different initialization logic for linked
data authorities and non-linked data.  Further, the initialization logic
was locked away inside of a controller.

With this commit, we expose a method that provides consistent
initialization logic; albeit with different returned classes that have
slightly different implementation nuances.  The application can then
rely on that and we can share initialization logic across controllers
and also expose a method that allows non-controller initialization to
begin to follow the same logic pathway.

Where are the tests?  The initializations are covered by controller
tests, so I have chosen not to write new ones.

There is further work to do because LinkedData and non-LinkedData there
are different signatures for `find` and `search`.  Ideally, we would
normalize the public facing implementation logic.  However, this commit
preserves backwards compatibility, simply introducing a new feature.

Prior to this commit, the returned value from `Qa.authority_for` was
something that was inconsistent.  Depending on the controller (or authority), the `find`
and `search` would require different parameters.

With this commit, those nuances are encapsulated in their own methods.
This begins to work towards a more foundational promise of questioning
authority; namely that the end point that asked questions of QA didn't
need to know which type of authority it was.

That promise was partially delivered at the routes level but this commit
pushes that down to the internal API level.

As a bonus, and perhaps confusion, I've included the
`Qa::AuthorityRequestContext` as an interface to help convey how one
might provide a different (non-controller) context to instantiating and
authority.

Prior to this commit, we always assumed that each "find" would request
from `https://id.loc.gov/authorities`.

With this commit, we use the "subauthority" to inform what the base URL
is for the `https://id.loc.gov/` host.  In some cases this is
`authorities` and in others it's as the code indicates.

Hopefully works to close the following:

- scientist-softserv/utk-hyku#267