Add language names to Source/Target labels in annotation UI

[cfedermann]

Is your feature request related to a problem? Please describe.
Sometimes it seems that annotators score non-translated text (in the source language, not in the target language) very highly. We assume that this may be due to the fact that they forget that the candidate text output should actually be in the target language.

Describe the solution you'd like
Adding the language names to Source/Target labels in the annotation UI will help make the different languages more explicit to the annotators.

Additional context
Should add this for all current annotation UIs.

[cfedermann]

The language pair under investigation is already visible on the top-right corner of the annotation UI. However, it feels advantageous to also add the language names to the Source/Target labels…

[zouharvi]

Looks like this:

[zouharvi]

Alternative is to have them in the box, which looks neater:

[zouharvi]

Resolved in #206.