Feature: "Add Projects"

[nerdstein]

I'm not fully clear on the best path forward, so I thought it might be best to describe the desired outcome. Please feel free to file subtasks for different components, events, or whatever.

The design reference shows the "Add Project" row under Advanced options: https://www.drupal.org/files/issues/2018-03-08/simplytest-ui.png

This is likely more than one component, as follows:

1. An autocomplete component for the project search
2. A branch selection based on the chosen project
3. An "Add More" button that copies the project/branch components in another row

There should be some sort of a limit incorporated (maybe an environment variable) that restricts the number of projects that can be added concurrently.