Empty states for issue lists and merge request lists

Background

Right now there are different empty states for issue lists and merge request lists for the project lists and group lists. In particular, the empty state is different if you are looking at a project with no issues, and if you are searching a project list but return zero results.

This is unnecessarily confusing and complicates the product (and thus the code). Let's simplify to improve stability and avoid situations such as: https://gitlab.com/gitlab-org/gitlab-ce/issues/31384.

I.e. we should not maintain these separate existing designs. They should merge into the same design.

Description

• A simple design to streamline everything into a single empty state.
• When you go to any issue (or mr) list, you always see the search bar (or filter dropdowns for group lists), even if there is nothing to show. There is an empty state message, there is a call-to-action (a button) to create a new issue (or merge request). And there is the email a new issue to this project (if it is a project list view).
• When you search an issue (or mr) list (or filter for group lists), and you get no results, you get the identical empty state with the message, call-to-action, and the email link (if applicable).
• In the case of the group list, the call-to-action button is a dropdown that lets you select a project.