Add support for `file://` URLs from home directory

As a snap user, I want to display files from my home directory with the file:// protocol.

Tasks

• Add home interface to snapcraft.yaml
• Expand validate_url() in configure hook to allow file://URLs – if home interface is connected.
• Test if WPE WebKit is already configured to load file:// URLs. If not, check which build flags or else need to be toggled.