Move UI export feature to its own step in form to allow for multiple formats in the future

This is the version that still exports.

The experience is as follows:

1. Dedicated HTML and ASCII export buttons replaced at bottom of main form with one button that goes to the next step in the form:

2. (Single export is removed from bottom of popup, people can do the single export by selecting a single project)

3. The next step of export is to pick a format, this allows for extending export formats and unify UI and drush:

4. If no project was selected to export, you get this error message and go back button: