Use the id anchor from collection-meta.yml

[hjanott]

Currently there is an anchor for the id field in the collection-meta.yml which is currently unused. Before the introduction of the split yml this was used.
Let's use this in the other yml files. Either by concatenating the string or yml objects of each collection yml with the collection-meta.yml in the services that load the yml information.

[vkrasnovyd]

See also #411 (comment).

[hjanott]

I also think we should do this for meeting_id. Currently all look similar to this.

  meeting_id:
    type: relation
    reference: meeting
    to: meeting/XXX
    required: true
    restriction_mode: A
    constant: true

They could then be extended by to and if need be restriction_mode and other keys.
Some didn't use constant true where I don't know if that's intended. For instance structure_level and structure_level_list_of_speakers.