i18n: Many docs and a few message improvments

Message documentation (qqq.json) should not just repeat the message content, that makes no sense as the translators see the message anyway; it should document the context in which the message appears, its eventual parameters and anything relevant for translators.

Some English messages contained spelling and typographic mistakes, and two were outright wrong (copy-paste errors), fix those.

In some cases, qqq.json contained different keys than en.json. In all these cases, en.json was correct, so fix the qqq.json ones. To find all these discrepancies, I used

diff --report-identical-files <(jq keys en.json) <(jq keys qqq.json)

in the front-end/src/i18n directory until it reported

Files /dev/fd/X and /dev/fd/Y are identical

(with X and Y being random numbers).