Add Tchap Configuration + Account Linking - Check if email is authorized by Tchap

[mcalinghee]

• ability to define tchap specific configuration in config.yaml
• refactor if email is valid by Tchap
• move identity server url to tchap configuration in config.yaml

tchap:
  identity_server_url: "http://localhost:8090"

[odelcroi]

To facilitate further merge, it would be good to add the tag //:tchap: like in the clients when adding to the upstream code.