Load all required font weights for Open Sans

Right now, we're only loading the default weight for Open Sans (400), which renders using css properties like font-weight useless. As a workaround, people have been using text-shadow to simulate a bolder font:

text-shadow: -0.1px 0 rgba(0, 0, 0, 0.4), 0 0.1px rgba(0, 0, 0, 0.4), 0.1px 0 rgba(0, 0, 0, 0.4), 0 -0.1px rgba(0, 0, 0, 0.4);

We should load the other relevant weights on base.html:

<link href="https://fonts.googleapis.com/css?family=Open+Sans:300,400,600,700" rel="stylesheet">

However, this will require a review of all the stylings that might be affected, as well as all usages to text-shadow to simulate bold fonts.