Closes #94 - element layout on small window sizes

[KelseyRM]

Related Issue/Keyword: Resolves #94 Description: This modifies how the add repository panel is laid out so that when the window size is decreased the elements don't jump around illogically.

This is done using flex boxes, and removes unnecessary styling classes.

• [x] Latest master merged/rebased into your feature branch
• [] Tests covering all changes
• [x] Meets the projects coding conventions
• [x] No out of scope changes
• [] @Mentioned any relevant team members
• [] Included a relevant gif (search here for inspiration)

[KelseyRM]

Testing was not initially done as there was no test suite in place, and the layout has been modified since then, but for this PR manual testing is as follows:

1. Navigate to the main screen, either by logging in or selecting 'continue without sign in'
2. Shrink the window beyond the clone / open buttons You should see the elements no longer jump around illogically, but stay underneath their respective header.