Revert "Different themes can be specified via CSS_THEME_URL env var."

This reverts commit e18eb423d963c1178da0934f28b3379489dbd478.

Reverting because we'll need plenty more ways to customize than just
CSS overrides, as CSOL needs a completely custom footer. There's
probably a more extensible solution that involves specifying a different
Jinja template rather than a separate style sheet.
Implemented a theme system that allows for custom HTML/CSS/JS content.

Themes are directories that contain `static` and `views` directories.
The `static` directory is mounted at `/theme/`, and any templates in the
`views` directory override Aestimia's standard templates. The primary
one to override is `layout.html`, which should extend `base-layout.html`.
This file can define its own `theme_head` and `theme_footer` blocks;
we can always add more blocks to `base-layout.html` if we need to make
it possible for themes to override other parts of pages.

I haven't yet documented this fully in README.md, because I wanted to
try out the system for a bit before investing too much into it, but
it seems decent and extensible enough.
rename dot_min app local var to DOT_MIN, as it's a constant.
Made the app name an app local var, rather than hardcoding it to Aestimia.

This will allow us to make the app name customizeable.
Themes can add to app.locals through app-locals.json in their root dir.

In particular, this allows themes to override APP_NAME, which is
what the CSOL theme now does.
gave docs.html a page title.
Added jquery.timeago plugin.
Times/dates on web pages are shown in timeago format, e.g. "3 hours ago".

Fixes #24.
fixed broken travis test, hopefully. forsaken date formatting.