Allow attachments on meetings for agenda items

[oliverguenther]

• Introduces a meetings API so it can be plugged into the attachable mixin for uploading files, as agenda items are not journalized itself, but only in the meeting container
• Extend the primer rich text input so that it can receive a resource

https://community.openproject.org/work_packages/26819

[dombesz]

LGTM, just a few minor comments. I tried to check the documentation at the /api/docs and the newly added api information do not show up, probably we need to add them to the dev/docs/api/apiv3/openapi-spec.yml.