Use transifex for i18n

[Changaco]

For #2642.

Translations are synced between git and transifex in release.sh. The Transifex password is automatically pulled from the Heroku environment variables when necessary.

I've temporarily removed the Chinese translation because it's very incomplete.

[rohitpaulk]

I know this is no big deal, but is PROJECT a variable that should be updated at transifex?

[Changaco]

IIRC this comment is generated by Babel, not Transifex.

[chadwhitacre]

Tried running the new release.sh and hit this:

writing PO template file to i18n/core.pot
env/bin/tx push -s
/Users/whit537/.transifexrc not found.
Pushing translations for resource gratipay-com.core:
Pushing source file (i18n/core.pot)
Done.
rm i18n/*.pot
env/bin/tx pull -a -f --mode=reviewed --minimum-perc=50
Pulling translations for resource gratipay-com.core (source: i18n/core.pot)
 -> eo: i18n/core/eo.po
 -> ko: i18n/core/ko.po
 -> fr: i18n/core/fr.po
Done.
sed -r -e '/^"POT?-[^-]+-Date: /d' -i i18n/*/*.po
sed: illegal option -- r
usage: sed script [-Ealn] [-i extension] [file ...]
       sed [-Ealn] [-i extension] [-e script] ... [-f script_file] ... [file ...]
make: *** [i18n_download] Error 1
[gratipay] $

[Changaco]

Fixed in 31b5ea7.