Use content script to insert style tag with fetched CSS.

This is to give fetched CSS greater importance than inserting as a
user agent stylesheet.
Add incuna styles.
Move tracking of decked tabs to content script for each tab.

Styles are now added as inline style element, meaning we can fetch
files that are sent as text/plain, which includes those from
raw.github.com.

Gistdeck script depends on jQuery, so we load that before this and
include it in the extension.