Subscriptions not creating posts for reopening PRs

[mickmister]

The subscriptions feature supports subscribing to different events of PRs and issues. One event that is missing for PRs is for creating posts for reopened PRs. Here is the current behavior of the plugin:

• [x] Reopening PR creates subscription post (it does not at the moment) Edit: now it does per https://github.com/mattermost/mattermost-plugin-github/pull/718
• [x] Reopening PR creates DM notification "user reopened your PR"
• [x] Reopening issue creates subscription post
• [x] Reopening issue creates DM notification "user reopened your issue"

The task here is to implement the feature for creating subscription posts for reopened PRs