Expand API to return available source and target languages

This is necessary to detach the web interface from /slang/tlang/ URLs in
favor of a global / or /search/ URL.

Note that the languages are cached on the first run to avoid hitting the
database again. The languages are alphabetically ordered.
Web UI: Do not use languages on URL for web interface

Now instead of having /en/fr/unit or /en/gl/unit we only have a / URL
for all the possible languages combinations.

This will allow to serve the web UI as static content in a future.

The languages in the dropdowns are automatically populated using
the available source and target languages in the database.
Web UI: Select default languages in dropdowns

Try to select by default the 'en' source language (if present) and also
try to select the browser preferred language as target language (if
present).
Web UI: Just hide the table headers and not all the table