Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Due to performance reasons we cannot use the type filter on the tags.
The table for ActsAsTaggableOn is too big and too unoptimized, such that
the queries time out on production.

See the discussion
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19740#note_120087938
for more info.

the potential number of available runner tags is too large to load it
statically to a dropdown. we use the same lazy loaded dropdown as is
used for the users dropdown already.

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".