Dev notifications config for failed and failing builds

[thatportugueseguy]

Description of the task

Extends monorobot config to allow for granular control on failed and failing builds DM notifications.

Adds a new key to the config: notifications_configs, where the users can add their own desired configs and commit to update it in real time, since the config file lives in the repo and not on the server.

Options are:

• dm_for_failing_build: defaults to true (to match current status rules). Determines wether a developer gets a message for a failing build.
• dm_after_failed_build: defaults to false (to match current status rules). Determines wether a developer will get a dm after a failed build. This is independent of the failed_builds_channel config.

In the end i didn't add tests for this, as it would implicate a lot of overhead for the small (and not complex) change, but maybe in the future we need to consider adding some unit tests with mocks and/or a way to be able to have a default test .monorobot.json config and the ability to have custom ones where it is relevant.