Tweak appearance

I set SimpleForm to use Bootstrap’s form styles, makes things look a bit nicer. I also fixed the position of the search buttons in the header and tweaked various small things.

I extracted some sets of styles from the monolithic `application.css.scss`; it’s sometimes helpful to break things out into separate files based on what they apply to, otherwise the main stylesheet file becomes overwhelming to work with.

Here’s a screenshot of the submission form after these changes:
![google chromescreensnapz001](https://cloud.githubusercontent.com/assets/43280/2624086/5d74f652-bd1f-11e3-84db-178362037333.png)

If people are more partial to Bootstrap’s [horizontal form](http://getbootstrap.com/css/#forms-horizontal), it should be possible to get SimpleForm to generate that instead.