Implements TemplateDropdown class to create custom template dropdowns ## What does this MR do? Refactorize template dropdowns. This MR creates a base TemplateSelector class so it can be reused for multiple types of templates. ## Does this MR meet the acceptance criteria? - [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added - [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) - [ ] API support added - [ ] Tests - [ ] Added for this feature/bug - [x] All builds are passing - [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides) - [x] Branch has no merge conflicts with `master` (if you do - rebase it please) - [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits) See merge request !4697