Refs #277: Some fixes after validating HTML and CSS

Things were really in quite good shape.

I validated HTML on a number of pages - the welcome page and front page
for most of the contrib apps - by load the pages and copying the page
source into the W3C validator. That found two problems: one form with
an empty action attribute, which I changed to "#" (picky), and an
input field of type "file" with a size attribute, which is not allowed
but apparently was emitted by Django's form renderer.

I pasted each of our css files into the css validator. base.css was
the only one with problems. I changed "border-color: none" to
"border-style: none" which I guessed was the intention, and deleted
a couple of lines that started with "*", probably left over from
incompletely deleting some commented-out code.

More problematic were some lines like this:

     filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff31c0d0', endColorstr='#ff2894c9', GradientType=0);

which gave the validator fits. I didn't know what they were and
so left them.

Branch: 277-validation
Merge pull request #277 from rapidsms/277-validation

Run HTML & CSS Validator
Merge remote-tracking branch 'remotes/origin/release-0.12.0' into feature/294-headers

Conflicts:
	rapidsms/contrib/httptester/templates/httptester/index.html

Branch: feature/294-headers
Refs #294: Fix registration side menu, headers

- Stop registration side menu text from being white-on-white by
  making the selector we use to make the responsive menu text white
  to be more selective.
- Noticed only one of the three registration pages had a header,
  fixed that while I was here.

Branch: feature/294-headers