Consolidate epic tree buttons (add/create epics and issues)

Problem to solve

We currently have two buttons the perform 4 actions in the epic tree header. Consolidating these will help reduce space needed, optimizing the tree for smaller screens. Listing all options within one button dropdown will also increase discoverability of all adding and creating options.

Further details

This will also free up space for other options such as expand all and collapse all

Proposal

The two add or create buttons are now just one dropdown button titled Add. When a user clicks this they are able to add or create epics or icons from the dropdown, which triggers the add or create flow we currently have for the tree.

Documentation

https://docs.gitlab.com/ee/user/group/epics/#creating-an-epic

Availability & Testing

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

What is the type of buyer?

Is this a cross-stage feature?

Links / references

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