* Actions: add Repo gardening action
More details about the action here:
https://github.com/Automattic/action-repo-gardening
The action is already used in the Calypso and Jetpack repos.
I've enabled 5 of the tasks the action offers so far:
1. Assign Issues (`assignIssues`): Adds assignee for issues which are being worked on, and adds the "In Progress" label.
2. Clean Labels (`cleanLabels`): Removes Status labels once a PR has been merged.
3. Flag OSS (`flagOss`): flags entries by external contributors, adds an "OSS Citizen" label to the PR, and sends a Slack message to the #themes channel.
4. Triage New Issues (`triageNewIssues`): Adds labels to new issues based on issue content.
5. Gather support references (`gatherSupportReferences`): Adds a new comment with a list of all support references on the issue. More details about this in pcLjiI-Rb-p2
* Add new Issue template for bug reports
* Enable newer tasks
- Prompt to reply to customers when an issue is finally closed after having gathered a lot of tickets. Reference: pcLjiI-Ro-p2
- Slack message when an opened issue starts gathering a lot of tickets. Reference: pciE2j-1iy-p2
* Remove unneeded tasks for now.
