Partially addresses #47424.

One should really use a separate table instead of using polymorphic
associations.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11168 for
more information.

squashed - fixed label and milestone association problems, updated specs and refactored reader class a bit

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

In 8278b763 the default behaviour of annotation
has changes, which was causing a lot of noise in diffs. We decided in #17382
that it is better to get rid of the whole annotate gem, and instead let people
look at schema.rb for the columns in a table.

Fixes: #17382

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

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