Implement "one-click" integrations for access plugins

[r0mant]

Description

We have implemented Cloud-hosted version of the Slack plugin that uses OAuth2 authentication flow to allow users to connect Slack to their Teleport cluster and starts the plugin in the auth server.

We want to extend this to other access plugins and "beef up" our new Integrations page as well with additional easy-to-use integrations for services we have already built.

Most of the access plugins we have already support OAuth2 authorization token flow so we should be able to provide UX similar to the "inaugural" Slack plugin.

For other plugins which do not support OAuth2 and only support e.g. API keys (e.g. OpsGenie), we will allow users to provide the API key and store it in some secret store in the Cloud (e.g. Kubernetes secret).

As part of this work, we will also be migrating plugins to the Teleport repository following Slack's example.

Plan

## Phase 0: Slack (13.0)
- [ ] https://github.com/gravitational/cloud/pull/2678
- [ ] https://github.com/gravitational/cloud/issues/2506

## Phase 1 (13.1 - 13.2)
- [x] https://github.com/gravitational/teleport.e/issues/688 @mdwn 
- [x] PagerDuty @tcsc
- [x] Jira @tcsc 
- [x] Mattermost @kimlisa 
- [x] OpsGenie @EdwardDowling 

## Phase 2 (14.0)
- [x] Discord @tcsc
- [x] ServiceNow @EdwardDowling

## Phase 3 (14.1)
- [ ] MS Teams @tcsc 

[zmb3]

@tcsc are we good to check off Discord as completed here?