Add ability to specify F-E and B-E ports

[lindsaydbrin]

Resolve # NA

Make it easier to simultaneously run multiple instances of Azimuth locally by passing port numbers in back-end and front-end commands.

Description:

Launch back-end with:

make CFG=<path_to_config> PORT=<backend_port_number> launch.local

(where PORT is optional)

Launch front-end with:

REACT_APP_BACKEND_PORT=<backend_port_number> PORT=<frontend_port_number> yarn start 

(where both PORT variables are optional, and backend_port_number should match the one for the back-end 🙃)

Checklist:

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• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[lindsaydbrin]

@JosephMarinier I made it work without setting the default! I really wanted to be able to extract PORT_ARG into the poetry command as a naked command, but I couldn't figure that out and could only get the string in the command, so I ended up using eval and string concatenation. Feels less elegant, but it works. Do you have suggestions for anything more elegant?

[JosephMarinier]

Tiny comment, but that's good to go! 👍