Fix relative Filepaths

Created by: pbatroff

When copying mosaico templates to classic message templates, relative filepaths seem to fail, mainly social media icons. There was a discussion on this problem in the mosaico extension issue tracker, see 182 and 160.

I think a replication on the filterHtml funciton in UrlFilter.php in the mosaico extension can fix this. it replaces relative Paths with absolute paths, and as long as the template is available this should work.

I tested/experienced the problem with the versafix-1.1 template, but I'm fairly sure that it will happen with default versafix-1 template as well. I'll test this and update the issue.