Clarify coding style

It would be helpful if there was a defined coding style (and ideally also linting) for everything. I see there's a linter for sass but don't see anything that's linting or defining the coding style for the HTML and javascript sections.

An example of where I am particularly unsure of the coding style is the single and double quotes on the same line in the same js file [here](https://github.com/EFForg/starttls-frontend/blob/c61e995c862db017dfd284677d7958cb7a184f50/js/validate.js#L9)

Start by reviewing js/validate.js at the referenced line and the existing Sass linter mentioned in the issue. Determine how coding style and linting should cover the HTML and JavaScript sections; done means the chosen rules are documented and linting coverage is defined for both areas, including the inconsistent quote usage.

Written by the indexing model from the issue text.