Adds a select with which a project may be selected for each repo.

Adds a select in the repo header that lists all of the user's active projects. Selecting a project associates the project with the repo until another project is selected. These mappings are saved in chromes sync store.

~~I thought I'd submit this even though it's not 100% yet because it is very similar to pull #5. I will be making changes to try and make this pull request more compatible with it. Most notably, using `me.json?with_related_data=true` to get the list of projects.~~

I welcome any comments.