Featured issues

I started a featured Civic Issues feature.

We'd replace the embedded widget on the CfA homepage with the new embed of 
`http://codeforamerica.org/geeks/civicissues/widget/featured`

It pulls three featured issues. It grabs regular issues if there aren't three left. It doesn't have tests yet and I might not have time to write them before Summit.

### Tasks
- [ ] Write simple tests for this and CfAPI
- [ ] Merge this PR
- [ ] Merge the [featured-issues branch on the CfAPI](https://github.com/codeforamerica/cfapi/tree/featured-issues) too 
- [ ] Upgrade the db of the cfapi `heroku run python app.py db upgrade`
- [ ] Get this CfAPI ids of the GitHub Issues to feature. [Here is a gist](https://gist.github.com/ondrae/e2b04f4a00156f93a1ef) of the ones I like.
- [ ] Mark them as featured. They'll then show up in the /featured widget
- [ ] Replace CfA.org embed with this new one.

@migurski @pui I could use some help with this, if we think Jen or Tim will be promoting it.