Add 'count' field for object listing endpoints

[Repumba]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

Listing endpoints always limit number of returned objects to 10

What is the new behaviour?

When requesting list of objects, user can specify a new count field and set the limit manually.
If the field is not provided, value is set to 10 by default.

Maximal number of returned objects is limited in MWDB's configuration (default value is 1 000).

Test plan

1. Manually test API
2. I've added automatic tests

Closing issues

closes #754