Maintain consistent template var paths in 3.2 installer

Paths to templates and assets should not end with a slash as they currently do in the installer. The rest of phpBB does not append a trailing slash to template vars for paths.

All template var paths should be consistent throughout phpBB with respect to the omission of the trailing slash

Here are just a few found in the installer:
https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/install/controller/helper.php#L273-L276