Followup to #10702 where two new strings were hardcoded in the template.
## Changes
- added as translatable strings
- made the template use a variable to avoid making it a long unreadable
- an unrelated change where tests related to this area of the template are moved to a separate file
- they were created in Forgejo: c92b4b12c8, 192177fc88
## Testing
Testing translations of all individual strings is considered not needed.
Added test for tmpl logic which was missing and that I've touched.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11333
Reviewed-by: Luis Adame <luisadame@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
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