Tidy up social sign in for non-JS

Before:
![before](https://f.cloud.github.com/assets/481903/1542031/17243e40-4d41-11e3-81fd-0ef1a0725502.png)

After:
![after](https://f.cloud.github.com/assets/481903/1542034/1ec298e0-4d41-11e3-9577-0b2c73c733ea.png)

Also cleans up button outline and removes two magic numbers.