Add config for failed builds channel

[thatportugueseguy]

This PR adds a config for a failed builds channel. This is a channel where notifications for failed builds get posted with the new CI steps that were broken, with an @mention for the slack user who committed the changes that broke the CI.

Only newly broken CI steps get notified, so when a user's commit breaks a step that was already notified, it doesn't get notified again.

[Khady]

do you have a screenshot to show the final result?

[thatportugueseguy]

Actually, looking again at the screenshot, I'm left thinking that we can make the notification more minimal and in line with the changes in #162.. We don't need to have a duplicate commit message, and we don't need the author in the commit lie, since we are identifying them in the slack mention.

[Khady]

are steps broken the new failures or all the steps that failed in a given build?

[thatportugueseguy]

only new failures. No preexisting failures are notified. On build #159 in the screenshot above, there are three failing steps, but only the two new ones are notified, with the author of the current commit in mention.

[thatportugueseguy]

I've made the notification a bit more minimal, I think it looks good. I'm happy to merge now

[Khady]

should we also notify when the build gets fixed?

[Khady]

can you update the documentation please

[Khady]

I guess we should have made the notification of committer optional too

[Khady]

those notifications will ony be for the main branch right?

[Khady]

those notifications will ony be for the main branch right?

They are not. I think it's a problem