* migrate 17 strings related to repository migrations from INI to JSON
* changed templates to get rid of unhelpful prefix `repo`
* migrate 4 strings related to counters
* also changed templates to get rid of `repo`, but it had to be done anyway to use `TrPluralString`
* Unhardcode the header on migraiton type selector page, which I haven't noticed in https://codeberg.org/forgejo/forgejo/pulls/6795 or in two other PRs I did on this template since
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11879
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Reviewed-by: Beowulf <beowulf@beocode.eu>