only add a trailing slash to appUrl if it isn't already there

https://bugzilla.mozilla.org/show_bug.cgi?id=1097858
we cannot guarantee across all the apps we plug into this system whether or not their path value on the url will contain a trailing slash or not. The trailing slash must be included, however, for cases such as a login from the root of a domain "https:webmaker.org" where there's no trailing slash