Code Monitors: add code to execute slack webhooks
Created by: camdencheek
This adds the actual code to execute webhooks. Its footprint is a little larger than ideal because I also had to move the email code into the backend
package to avoid circular dependencies.
There is still no way to create new webhooks, so this code is not executed outside of tests.
Note that this isn't the copy-edited form of the slack message, so the actual message will be reviewed for content/visual design. This is mostly just a functional placeholder.