Make `Add issue` & `Add epic` from epic tree header searchable

Our current epic tree design adds the ability to create or add an epic/issue from within the epic page itself. The two options for each are "Add existing" or "Create new"

For add existing we only have the option to paste in a URL or ID

Screen_Shot_2019-06-27_at_1.41.33_PM

Let's update this to add a search dropdown of existing epics/issues, similar to https://gitlab.com/gitlab-org/gitlab-ee/issues/6878

You should be able to search by title, URL, or ID

cc @uhlexsis I think you had some ideas about this a few months back too 🤔