Refactor styling

[Rumperuu]

Currently, we have 8 CSS stylesheets in the css/ dir (ignoring the minified ones that are still tracked in version control for reasons:tm:). When building the Plugin for release, these are concatenated in various combinations into a bunch of footnotes-<code>.css files, which are then minified and linked conditionally based on user-defined settings (which are used to build the <code>).

There are a number of issues with this:

• the <code>s are hard-coded, meaning that build_stylesheets.sh must be updated if the user-defined settings ever change; and
• there is inconsistent namespacing in the stylesheets, causing conflicts such as this one (Version 2.5.11 and above disable widget areas in Hueman Theme #81) and this one.

As part of #87, I will also look into how we can reorganise this. For example, starting to refactor the stylesheets into SCSS to make namespacing more reliable, splitting components into individual files, etc. Once we have a proper CSS build pipeline in place, this should have the knock-on effect of removing the need for the build_stylesheets.sh command, as we'll be able to define much more fine-grained stylesheets to be conditionally imported (and, of course, we can roll in #88 as well).

[markcheret]

please, yes. SCSS sounds like a step in the right direction. Generally, any widely used and available templating, build and rollout friendly system... So yeah, I'm all for it.