Add service Todoist

[sduck]

[elliotchance]

Reviewed 5 of 5 files at r1. Review status: all files reviewed at latest revision, 3 unresolved discussions.

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src/OAuth/OAuth2/Service/Todoist.php, line 10 [r1] (raw file): Is this comment supposed to be here?

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src/OAuth/OAuth2/Service/Todoist.php, line 25 [r1] (raw file): /** is for docblocks of the following element, this is not. It should be /*.

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tests/Unit/OAuth2/Service/TodoistTest.php, line 10 [r1] (raw file): Is this comment supposed to be here?

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Comments from Reviewable