Extract notes to translators from comments in code

This extracts comments that contain @translators above the line
that calls a __*() function.
Remove unused code that contains unused strings
Move some remaining strings to admin domain
Allow the number of results string to be moved

after or before the title.
Remove useless string

that can be difficult to translate.
Use the printf %n$s syntax wherever params > 1

so that translators can reorder params.
Various i18n fixes.

Include markup as less as possible in translatable strings,
factorize code, fix markup, fix URLs refering to tatoeba.org,
merge translatable strings.