Allow users to search for Epics when adding sub-Epics

### Problem to solve Copy + pasting Epic links to add a related Epic into the widget is a cumbersome experience. I propose we match a user's expected workflow of searching for and selecting an Epic to add within the context of the widget itself without having to open a new page or tab.

Proposal

Some basic ideas were captured here, but a search experience is TBD. Ideally it will not stray too far from the experience built in https://gitlab.com/gitlab-org/gitlab-ee/issues/6878.

What does success look like, and how can we measure that?

Users are more quickly adding Epics to their widget without having to break their workflow and hunt for links in another page.

Links / references

Solution

User is prompted to search for epic

On click of the input, the user is able to access the epic dropdown and add multiple epics. The epics tokenize in the input as they add.

This looks a bit weird since the input is wide, kind of similar to the search/filter bar though I guess?

User sees the tokenized list of epics they would be adding

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