Support plugins extending Webpack build pipeline; Build translations from openproject-translations

This PR concerns itself with two related, but separate themes:

### Support plugins extending Webpack build pipeline
https://community.openproject.org/work_packages/5697
https://community.openproject.org/work_packages/17241

###  Build translations from openproject-translations, if available
https://community.openproject.org/work_packages/14679

See also 
* finnlabs/openproject-backlogs#114
* finnlabs/openproject-costs#115
Redefinition of '_'.