Changes for review: Allow unused categories and tags

[jimafisk]

@roobyz I meant to create a PR for this, but I forgot that I was pushing directly to this repo. I made a couple of changes I want you to be aware of:

1. I updated .gitignore so we're tracking the themes dir: fe501df. The hope is that you track theme files with your project instead of using a git submodule. Should clear up confusion like I noted here: Deploy failed in build step jamestagal/edtechdesigner#29 (comment)
2. There was a bug in the theme where if you created a catgs or tags that wasn't used in posts content that it would break the build as discussed in this issue: Deploy failed in build step jamestagal/edtechdesigner#29. In Ben's case, there was a corresponding posts JSON content source in the project with the same name as every original post in the theme, but the theme was still defining catgs and tags in its content source that were no longer being used (since all the posts in the theme were overridden). I pushed a fix that should allow the build to complete, but it might not ultimately be the way you want to handle this: 7dd5f50