Install git hooks as part of prepare-dev.py
Install git hooks as part of prepare-dev.py

We now install a `post-checkout` git hook as part of `prepare-dev.py`
that will remove all `.pyc` files in the repository when a new ref is
checked out. This is to prevent stale `.pyc` files from other release
branches from sticking around when working on patches on multiple
versions of Review Board (without having to provide instructions on
setting up multiple `git worktree`s to developers).

Testing Done:
Ran `./contrib/internal/prepare-dev.py` and observed the following:

- if the hook did not exist it would be created;
- if the hook did exist but didn't match the content a warning was
  printed; and
- if the hook did exist but was non-executable it was overwritten.

Reviewed at https://reviews.reviewboard.org/r/10182/

reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard