Tidy up "pkg_mgr_installed.php" XHTML

Add closehead variable and move scripts into HEAD and manually close
HEAD
Add missing close FONT tag
Close BR tags
Deprecate ampersand in ANCHOR tags
Change target in ANCHOR tags to "_blank"
Add ALT statement and close IMG tags
Merge pull request #522 from ExolonDX/branch_16

Tidy up "pkg_mgr_installed.php" XHTML