Split Revision class to support Phabricator & Github

[La0]

Part of #2680

In order to support new code-review ending tasks started from Github+Lando (example), we need to split the bot's Revision class into:

• Revision as a base class defining a shared interface
• PhabricatorRevision which inherits from Revision and implements all the code related to Phabricator
• later on, we'll add GithubRevision

You can start from this commit which already does the split, then remove anything related to GithubRevision as it will be introduced in a dedicated followup.

You'll then need to replace every mention of Revision by PhabricatorRevision in the unit tests.