Introduce a Todo presenter

Todo helper contains many methods which would be a good fit for Todo presenter, to clean-up this helper we should rather create Todo presenter and move these methods there.

Follow-up issue from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30435:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30435#note_204540150 - We should move the helper methods from app/helpers/todos_helper.rb to a presenter object.

/cc @jprovaznik