Modernize prepare-dev.py
Modernize prepare-dev.py

Our prepare-dev.py was doing quite a few things that weren't best
practices:

* Global mutable state in the form of `options`.
* Using the deprecated `optparse` module instead of `argparse`.
* No docstrings anywhere.

These issues have all been addressed. Where we were previously using
global mutable state we now pass our state as an argument to the
function that requires it (`create_settings`). Our argument parser has
been updated to use `argparse.ArgumentParser`. The module and all
functions now have docstrings that meet our documentation guidelines.

Additionally, I cleaned up control flow a bit in `main()` where things
were running in a `try..except` that didn't need to.

Testing Done:
Ran `prepare-dev.py` and it ran correctly.

Reviewed at https://reviews.reviewboard.org/r/10179/

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