Support PKCE (Proof Key for Code Exchange) by OAuth clients

According to https://tools.ietf.org/html/draft-ietf-oauth-security-topics-13#section-3.1.1 authorization servers MUST support PKCE to prevent authorization code interception attacks.

Since our authorization codes are not stateless, this requires a schema change to add two columns to the editor_oauth_token table: code_challenge of type TEXT and code_challenge_method of type oauth_code_challenge_method.

oauth_code_challenge_method is an ENUM of ('plain', 'S256').
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