UI refresh

This is a partial pull request for the new UI (#329) so that others can get to work with what I've done. Most notably:

Here are things to note:

* This is only for the source interface, I haven't changed the design for the journalist interface yet
* I made a new template, `source_templates/index.html`, which doesn't use `base.html` because the index page has a different layout now.
* I made the flash notifications look way better.
* This also fixes #342.

Here are things that aren't yet done in the source interface:

* Custom header images (in `securedrop/config/custom_theme_image.py`) don't work anymore, right now I've just hard-coded the SecureDrop logo there. There's commented out code at the bottom of `source_templates/base.html` for using the custom header images, but I haven't made it work yet. 
* When a source receives a reply from a journalist, the reply box looks like crap. I didn't get to making the reply box look good. Also it looks like the "You have received a reply. For your security, please delete all replies when you're done with them." notification box doesn't have padding or something.

I think that's it. All the tests pass too. (Maybe we should write a custom header test?)