Adjusted label-matching to be case insensitive

[xinyuluo]

Description of the task

Adjusted label-matching to be case insensitive.

How to test

In mock_payloads, one can find a newly added .json files called pull_request.closed_capitalized.json.

To test that label-matching is now case-insensitive, run the following commands:

./notabot check --json path/to/file

The expected output should be:

Using push routing:  
  backend/api* -> #aa
  backend/megaindex* or backend/ahrefskit* -> #backend
  any -> #all-push-events
Using pull request routing:
  backend* -> #backend
  aa* -> #aa-git
  siren* -> #siren
  not backend* or aa* or siren* -> #frontend-bot

As the label matching is now case-insensitive and the PR in this test has label "AA", one should expect the notabot to send notifications to #aa-git. If label matching is still case-sensitive, notabot will send notifications to #frontend-bot.

[Khady]

@xinyuluo I have added a test setup in #27. So now all the json files in the mock_payloads dir are tested and the output of notabot is captured. When you add a new payload, you must run make test_promote to capture the new output. You then check that the diff is expected. And eventually commit it.