[Feature] 8720 toolbar at bottom of details view

[``* `#8720` Toolbar at bottom detail view``](https://www.openproject.org/work_packages/8720)

:exclamation: https://github.com/finnlabs/openproject-costs/pull/104
@hschink where links are to "classic" HTML pages and not to other API resources, I think we should specify the MIME type, i.e. `type: 'text/html'`. See http://tools.ietf.org/html/draft-kelly-json-hal-06#section-5.3