Fix: url helpers outside request context

WP [#4797](https://www.openproject.org/work_packages/4797)
This is a presumably preferable alternative to PR [#941](https://github.com/opf/openproject/pull/941) or [subdirectory routing](https://github.com/opf/openproject/tree/feature/subdirectory_routing).

This PR fixes all known spots in the core where URL helpers are used with no request available through context. Specifically code under models.

It does so by introducing so called static routing URL helpers which can be safely used even within models.

There are still a number of plugins that have to be adjusted to use those helpers as opposed to calling `Rails.application.routes.url_helpers`, which require a request to be available, directly.
Those plugins are:

* openproject-costs (PR [#85](https://github.com/finnlabs/openproject-costs/pull/85))
* openproject-documents (PR [#12](https://github.com/finnlabs/openproject-documents/pull/12))
* openproject-meeting (PR [#38](https://github.com/finnlabs/openproject-meeting/pull/38))

Possible changelog entry:
<pre>* `#4797` [Subdirectory] Broken Links</pre>