Allow arbitrary username

[hovissimo]

This PR removes the requirement that the user select 'pifinder' as the username for their Raspberry pi.

Overview of changes:

• updates hard-coded user dir paths with $HOME
• moves pifinder.service to pifinder.service.template and updates the install script to interpolate that template back to pifinder.service with the current user's username
• One incidental change updating astro_data_dir to a consistent reference to utils

I tested these changes by installing PiFinder on a new sd card with my own username.

[hovissimo]

This resolves https://github.com/brickbots/PiFinder/issues/100

[brickbots]

Thank you @hovissimo for the contribution, it will come in useful in the future. I'm closing this particular PR as we are starting to move to a new OS release (bookworm) and some of this will need to be re-worked a bit.