[meta] Implement Basic Quality Estimation

[andrenatal]

We need to be able to enable the quality estimation functionality by switching a pref. This same pref should also disable the functionality that causes performance penalty in the engine, and not only the UI feature.

List of tasks:

• Integrate Basic Quality Estimation #132
• Modify extension to encode plain text and send it as HTML to the engine whenever QE feature is on
  • This is required as engine always expects html text to return QE scores (as per this)
  • The translation doesn't need to be decoded back as the engine always returns translation as html whenever QE is on and this output can be used directly to show colors
• Switch Quality Estimation specific model config off when not in use in order to have better performance #133
• Show colors for in-page translation from the quality scores returned by the engine #179

[andrenatal]

Closing this since all references items were closed.