Issue #3619989: Replace the placeholder of a failed translation with the text it stood for

The module was written in one language and translated to English in a single commit. Where that pass produced nothing it wrote the word standing in for the text, and 402 of those survived in 28 files: in comments and docblocks, but also in the arguments of t() that reach an API client, in the subject of a submission e-mail, and in the label and page name of several REST plugin annotations.

The text is restored from the commit that precedes the translation pass and translated properly. Five of the lines were ambiguous - the same skeleton stood for different originals - and were resolved from their surroundings rather than from the first candidate: the page resource is a simple page, not the front page; the line in the facets builder is about removing language prefixes, not about view logs; the one in the doctor form introduces the problem description field.

A unit test now reads the module tree and fails if the placeholder comes back. Neither PHPCS nor PHPStan reports it, which is why nothing caught it for five months.

Closes #3619989