Fellowship gov recruit content reorganization: Iteration 2

This PR changes some small things to continue to smooth out the fellowship city/gov recruitment flow.

* Updates the text on the homepage with suggestions from @ZoeBlumenfeld — [See that page](http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/fellowship-reorg-2/)
* Changes link off homepage to go directly to /governments/fellowship — [Same page](http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/fellowship-reorg-2/)

^ @Jaime-Alexis: Please review this content, it's the words in the homepage header.

* Adds simple JavaScript form validation to our interest form to check if required fields are filled, email is input in a relatively sane format — [See the form page](http://jekit.codeforamerica.org/codeforamerica/codeforamerica.org/fellowship-reorg-2/forms/governments/fellowship/interest/)

^ @migurski or @phae: could you check out my JS here when you have a second? Please check to make sure my syntax isn't too wonky, my testing shows it should work as expected on paths. (Advise using Safari to do path testing, as Chrome is smart and will tell you to fill the fields in before it falls down to the JavaScript.)
You might add an extra `<br>` doing it this way, but nbd to me. Another way would be to make a list of invalid messages, and then join them together with `<br/>` (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/join). But this is totally good.