Lots of i18n enhancements, updated German translations

Greatly improve i18n stuff:
- unified textdomain (to the one and only 'buddypress-media')
- added missing gettext syntax where needed
- simplied some strings using sprintf() and placeholder principle
- freed some strings from markup
- updated default .po/.pot file for translators (including above additions)
- updated & improved German translations (including above additions)

Note: NONE of your actual strings/wording was changed, only some of the syntax to make things more in line with WordPress coding standards and make work for translators a bit easier.

Thanks, Dave from Germany :)