Allow for multiple labels with the same starting letter on changes.meta

[joao-paulo-parity]

• Changes the format of changes.meta such that multiple labels with the same starting letter are taken into account (related to Handle non mutually-exclusive labels #9)
  Check out the new format in the diff of test/test_meta.rb
• Pulls compute_change_meta and parse_change_label out of Changelogerator so that they can be unit-tested easily
• Make it possible to run unit tests locally in the absence of $GITHUB_TOKEN

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I added some instructions for how to try this out in paritytech/polkadot#6702

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closes #9