Add option to hide status message when no automerge label

[paescuj]

• Add option to hide status message on PR's until automerge label has been added, closes #747 (option defaults to true, so no breaking change)
• Make the missing automerge label message a little more friendly (see https://github.com/chdsbd/kodiak/issues/747#issuecomment-958260341)
• Add tests for new option and fix other tests

I've tested this via a self hosted Kodiak instance:

First scenario:

1. Create a PR (don't add automerge label)
2. Verify the correct message (Ignored (no automerge label: ...) is shown on PR

Second scenario:

1. Set missing_automerge_label_message to false
2. Create a PR (don't add automerge label)
3. Verify no message (Ignored (no automerge label: ...) is shown on PR

Third scenario (addressing https://github.com/chdsbd/kodiak/issues/747#issuecomment-958565441):

1. Set missing_automerge_label_message to false
2. Create a PR (add automerge label)
3. Verify the correct message (merging PR (waiting for status checks ...) is shown on PR
4. Remove automerge label before checks have been finished
5. Verify the correct message (Ignored (no automerge label: ...) is shown on PR

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[paescuj]

This change looks great!

One minor suggestion about changing the config name

Thank you for your quick review!

I've updated the config name.