Web UI: Add new web interface

This provides a new web interface:

- Styled using Bootstrap,
- With links to Translate House's major products,
- Modal dialogs for help, about, data sources and contact,
- Basic JavaScript code to query the amaGama API directly and display the
  results without reloading the page, using a minimum similarity of 50 and
  displaying at most 20 results.
- Also includes hidden tabs for when amaGama supports more way to query it,
- Remove Flask-WTF and WTForms requirements, because the results are now
  retrieved using a call to the API through JavaScript,
- The view only replies to GET requests now, since we don't need to handle
  the form.