Replace the current toolbar implementation with the new PageHeader component on the (rails) admin pages

* [ ] Overview
* [x] Users and permissions, will be done in the scope <mention class="mention" data-id="50109" data-type="work_package" data-text="#50109">#50109</mention>&nbsp;
* [x] (and child pages)
* [x] Work packages, will be done in the scope of <mention class="mention" data-id="56585" data-type="work_package" data-text="#56585">#56585</mention>
* [x] (and child pages)
* [x] Projects, will be done in the scope of #56586
* [x] (and child pages)
* [ ] Custom fields
* [ ] Attribute help texts
* [ ] Enumerations
* [x] Calendars and dates, will be done in the scope of #56587
* [x] (and child pages)
* [x] System settings, will be done in the scope of #56587
* [x] (and child pages)
* [x] Email, will be done in the scope of #56614
* [x] (and child pages)
* [x] API and webhooks, will be done in the scope of #56587
* [x] (and child pages)
* [ ] Authentication, will be done in the scope of #56615
* [ ] (and child pages)
* [ ] Announcements
* [ ] Design
* [ ] Colors
* [ ] Enterprise edition
* [x] Time and costs, will be done in the scope #50109
* [x] (and child pages)
* [ ] Backlogs
* [ ] (and child pages)
* [x] Github integration, will be done in the scope of #56587
* [x] Storages
* [x] (and child pages)
* [ ] Billing
* Begin of the insertion[x]End of the insertion Begin of the deletion [ ]End of the deletion Slack integration, will be done in https://github.com/opf/openproject-slack/pull/10
* [ ] Plugins
* [ ] backup
* [ ] Information