Environments dashboard: Improve the `Add project` modal

Problem to solve

We are introducing a dashboard view for environments on https://gitlab.com/gitlab-org/gitlab-ee/issues/3713. Part of the user flow includes adding projects to the dashboard using a modal. At this point (MVC), the usability Add Projects modal is very vague and does not help users to clearly see what to add there.

Further details and proposal

Add the same UI text that will be updated in: #12870 (closed)

Out of scope

User needs to know ahead the name of projects in order to find them using the autocomplete search input. We can help users by providing them indication of their recent or starred projects in the modal, so they can just click and add it to the dashboard without having to search.