WIP: [i18] Add context to 'donate'

[jorgesumle]

Aims to solve #972

It's a matter of using contexts where necessary. I think it would be reasonable to reuse the existing translation of "donate" (in languages where it's translated) for these new strings (because these 3 new contexts mean new untranslated msgids in the .po file).

[Changaco]

For this to work the pgettext function would have to be implemented in liberapay/i18n/base.py and injected into the context in the same way as _ and ngettext.