Add slack notification for all ETS cron jobs

[rahulporuri]

At the moment, we need to manually check the status of the cron jobs across the ETS repositories, which is time consuming and error prone. It would be better if we could simply setup slack notifications for all ETS cron jobs.

• traits
• pyface
• traitsui
• envisage
• enable
• chaco
• apptools
• traits-futures

[rahulporuri]

I added a secret to all actively maintained ETS repos (minus scimath) for the slack channel ID.

[rahulporuri]

closing for now as notifications have been setup for all active projects - except scimath. There are some issues with the scimath setup - which will be resolved later.