Consider replacing local task runner with `invoke`

[lizadaly]

Local tasks like running builds or performing maintenance use the fabric3 library, which works well and developers are comfortable with. There are some reasons to consider switching tooling here though:

• fabric3 is at end-of-life, because the parent Fabric library is now under active development again.
• Fabric is primarily a remote code execution tool, but deployment is now handled in other ways. Requiring Fabric brings along a lot of heavyweight dependencies like paramiko that require version updates despite being unused.

Invoke is close to a drop-in replacement for Fabric when used for only local tasks. It has no dependencies.

(It might be convenient or it might be confusing but it would be possibly to retain fab as an alias so people can continue to use the commands they're accustomed to during development!)

[lizadaly]

Another, perhaps more streamlined alternative, is to just use custom Django management commands for tasks that use the Django environment and a simple makefile or bash script wrapper for anything else, avoiding additional dependencies entirely.