Extend resource statistics with backup quota

- ITACLOUD-3035
Cleanup text i18n.

- Stick to English in APIs/internal labels as much as possible.
Fix event formatter
Add backup quota/usage to resource stats
test corrected and filter added to membership queryset (itacloud-3035)
Merge branch 'develop' of code.opennodecloud.com:nodeconductor/nodeconductor into feature/itacloud-3035
Merge branch 'feature/itacloud-3035' into 'develop'

Feature/itacloud 3035 - backup_storage stats + i18n cleanup