Refactor AutocompleteController

This refactors the AutocompleteController according to the guidelines
and boundaries discussed in
https://gitlab.com/gitlab-org/gitlab-ce/issues/49653. Specifically,
ActiveRecord logic is moved to different finders, which are then used in
the controller. View logic in turn is moved to presenters, instead of
directly using ActiveRecord's "to_json" method.

The finder MoveToProjectFinder is also adjusted according to the
abstraction guidelines and boundaries, resulting in a much more simple
finder.

By using finders (and other abstractions) more actively, we can push a
lot of logic out of the controller. We also remove the need for various
"before_action" hooks, though this could be achieved without using
finders as well.

The various finders related to AutcompleteController have also been
moved into a namespace. This removes the need for calling everything
"AutocompleteSmurfFinder", instead you can use
"Autocomplete::SmurfFinder".