feat(dev): emit descriptions for commands in help

[miketheman]

Description

Help the developer understand what each target (subcommand) does.

Now looks like:

$ make
Call a specific subcommand:

serve                          Start the application
migrations                     Generate migrations from models
migrate                        Run Django migrate
manage                         Run Django manage to accept arbitrary arguments
shell                          Open Django interactive shell
clean                          Clean up the environment
test                           Run tests
docker_shell                   Open a bash shell in the web container