Add functionality for db-object-widget and select2

[kennsippell]

Alternative potential solution for #210

There is another solution for the issue at #232

[kennsippell]

The current status of this PR:

• Wires the db-object-widget via a fork of the select2-service from cht-core. Requires a few tweaks: ts to js, setting delay: 0
• Mega-MVP for the document which the select2 should load. Currently I just hardcode it as documentFoundBySelect2
• Non-existent search and refinement functionality as user types
• This has a draft of the changes required in form-filler. It doesn't work and I'm worried about nailing reliability.

I really can't tell if this is worth the maintenance challenge + the risk to test reliability. At this time, I'm inclined to shelf this PR and proceed with #232 and just disable the db-object-widget until there are clear scenarios where this is more useful.