Add support for setting context of strings in ooni/translations

[hellais]

It was reported that:

For the six "Blocked", "Tested" and "Working" strings, could you please add the explanation in the string description field since not every language team collaborators will read the comments/issues?

But because of how we set the comment field of the context of a string, this would be overwritten next time we do a sync.

See: https://github.com/ooni/translations/blob/master/tx-set-context.py#L59.

We should maybe add an additional column to the master CSV and include the context comment in there.

[hellais]

Closing this in light of #2845