Take into account that the user may already have the Allsky Website installed, so save it, then install the new version, and restore saved videos, keograms, and startrails as well as any associated thumbnails.  Also restore configuration files.
If the user is installing Allsky Website for the first time, set some configuration variables based on the Allsky configuration.

The code dealing with configuration files was moved to a function "modify_configuration_variables" in case we want to call it on "--update".

Add an error check.  Make error messages more consistent.
Improve message after completion and include Wiki URL for new website users.
Remove "set -x" left over from my testing.

functions.php needs to know if it's on a Pi so it can check the "XX_ALLSKY_CONFIG_XX" placeholder and if not updated, warn the user.
On remote servers we should NOT check the placeholder.  The "XX_ON_PI_XX" placeholder is set "true" during installation on a Pi which tells functions.php it's running on a Pi.

There is no longer a need to replace only the 1st instance of XX_ALLSKY_CONFIG_XX since there is now only one instance.