1. Fix the broken `gettext()` call in `courde_detail.js`
2. Change the string to avoid Transifex misunderstand the pattern '% o' in `checklist.underscore`.
3. Embed the `{filename}` part inside the string in `transcripts-choose.underscore` to provide better i18n flexibililty.
4. Remove all the call of `gettext()` and `${_()}` inside the `mock` directory to aovid mis-extraction by pybabel. (It seems that the `mock` directory is soly used in test? So the strings in this directory need not to be extracted and translated?)
5. The unicode char `\u2014` will trigger a bug inside pybabel which makes the extracted string untranslatable, so remove this char.