post_install.sh: Use uaccess tag in udev rules

[tormodvolden]

Do not make device node world-writeable, instead give access to desktop user.

Also use one "here" document instead of mixing in echo statements. Customarily keep "then" on same line as "if".

[CLAassistant]

All committers have signed the CLA.

[tormodvolden]

(fixed up commit message)

[tormodvolden]

Signed the CLA with the help of this: https://github.com/cla-assistant/cla-assistant/issues/730#issuecomment-913610724