Example:
* Files from the German Wikipedia contain `{{Bild-GFDL-Neu}}`.
* This should become {{GFDL|migration=not-eligible}}.
* The instruction for this is `;Bild-GFDL-Neu:GFDL|+migration=not-eligible`.
The `+` changes the meaning of what comes behind the `=`. It's not a source parameter name any more, but a static value. This means:
[x] CommonsHelperConfigParser already understands the `+` and turns it into a `addIfMissing` flag.
[] Change CommonsHelperConfigParser so it does not use `localParameters` if a `+` was present, but remembers a `value` instead.
[] Teach WikiTextContentCleaner to properly work with parameters that have a `value`:
[] Make sure parameters marked with `addIfMissing` are added to the template, if missing. For an MVP it's ok to add such parameters either before or after all other, previously existing parameters.
[] Make sure a parameter that already exists is not duplicated. Only the value should be replaced then.
Possible follow up tickets:
[] Make sure a new parameter is added at a position that closely matches the position in the configuration file. E.g. a configuration like `a=…|+b=…|c=…` and a source like `|a=…|c=…` should result in `a=…|b=…|c=…`.
[] T198582: Allow a configuration like `a=source_parameter|+a=fallback value` where a target parameter appears twice, one time with the name of a source parameter, and a second time with a static value. The static value should only be used if the source parameter was missing or empty.