Remove welcome.php and replace with filter of default content

WSUWP allows us to override the default content for the home page
rather than including a template
Remove the check for a default post ID and title
Correct the URL to customizer.png
Filter the default title when a site is created on WSUWP
Remove duplicate title and article element from default content
Filter the default home page template to use single.php
Merge pull request #65 from washingtonstateuniversity/update/initial-content

[PR] Use filters provided by WSUWP for configuring default home page content