Change locales file generation to use JSON sources (#2028)

* Change locales file generation to use JSON sources

Instead of inheriting in JS files, set locale inheritance in the
theme's YML file, and inherit in the generated locale file, rather
than the source file.

* Convert glitch-soc JS translation files to JSON

Obtained running the following:

```sh
sed -i -z "s/import inherited from '.*';\s*\nconst messages = //" *.js
sed -i "s/\s*\/\/.*//" *.js
sed -i -z "s/;\s*export default .*/\n/" *.js
for i in *.js; do
  json5 $i | json_pp > ${i}on;
done
```

* Change `yarn manage:translations` to exclude any translation already defined upstream

* Run yarn manage:translations

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