Reuse content-script-home for Firefox extension

Fixes the Firefox extension, which broke when content-script-home.js
became Chrome-only in de830599512edc3e238faa7a373f7647a31845b9. Moves
content-script-home.js out of the Chrome extension directory to make it
more apparent that the script is shared between multiple browser
extensions.

Releases a new version of the Firefox extension with support for
Altamira timecards.

Ports configurable training and book budget preferences from the Chrome
extension.

Closes #12