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

#### Notes for review:

* Webpack will not bundle plugins (and fail silently) if a `Gemfile.lock` is inconsistent (for example, if there are conflict makers). I'm not how/if we should make this more obvious.

* Shelling out from node to Ruby is currently done **synchronously**, which is not really _the node way_, but I wasn't really sure how to handle callbacks in the webpack config.

###  Build translations from openproject-translations, if available
https://community.openproject.org/work_packages/14679

#### Notes for review:

* ~~You can specify an `OPENPROJECT_TRANSLATIONS_ROOT` if you want to
  test this PR without adding openproject-translations as a dependency.~~

  _Removed in 4ea7cb0. Use a bundle override instead (e.g. `bundle config local.openproject-translations ~/git-repos/openproject-translations`_

---

See also 
* finnlabs/openproject-backlogs#114
* finnlabs/openproject-costs#115
Redefinition of '_'.