Support for Slack including login, notification and slash command

[Anan1225]

Is your feature request related to a problem? Please describe

It is used for further support of Answer in Slack.
Feature1: User authentication.
Feature2: Enable notification for selected questions and answers.
Feature3: Slash command for quick use of Answer in Slack directly.

Describe the solution you'd like

Using OAuth2.0, similar to WeCom UserCenter, to implement login. Enhance and integrate current notification plugin. Develop /ask and /search directly within Slack.

Describe alternatives you've considered

Develop a new notification function instead of leveraging current one which may be a redundant and unnecessary development work.

[Anan1225]

I am willing to take on this feature and implement the three functionalities step by step.