Move periodic checks to github actions

[jameesjohn]

Since probot scheduler is being archived, we need to move the periodic checks to Github actions.
Github actions allow us to set up a cron job as part of the jobs needed for the action. But to enable the action to be able to comment on pull requests, we need to authenticate as oppiabot.

See probot/probot#1370 (comment) for more information.

[seanlip]

Note: this may be related to #277 (but I'm not 100% sure).