Allow the ability to make fish the default shell for a specified user, and always add fish to the list of available shells when installing from source

Previously, there was no way to make fish the default shell for any user than the hardcoded "vagrant" user. These updates fix that. Additionally I changed guard statement for adding fish to the shell list when installed from source (details in the appropriate commit message), and made it so that fish is added to the shell list even when it's not being specified as a default shell for any user.