Add support for missing types of changes on the ProjectV2Item

[artcz]

We currently support changes in date fields (like start_date, end_date), and single choice fields (like status)

Would be great to add support for at least the following:

• Add support for changes to Assignees #18
• Add support for changes to Linked pull requests #17
• Add support for Labels #16

This can be added here: https://github.com/EuroPython/internal-bot/blob/main/intbot/core/integrations/github.py#L168-L199

And the tests for this are here: https://github.com/EuroPython/internal-bot/blob/main/intbot/tests/test_integrations/test_github.py#L182-L249

Would be also great to refactor that changes parsing code, but that could be something for later :)

The example of the webhooks with different types of data are in the comments.

[artcz]

On a second thought.. Looking at the content of the webhook, it might be easier to actually implement it on the issues themselves:

This is content of the webhook for changes in assignees

  "changes": {
    "field_value": {
      "field_node_id": "PVTF_lADOAFSD_s4AtxQyzgkedqE",
      "field_type": "assignees",
      "field_name": "Assignees",
      "project_number": 5
    }

This would require pulling more information from Github API.

Might be easier to implement "assigned", "unsassigned", "labeled", "unlabeled" on Issue...

See #5 for more ideas