Adds the ability to use blog slugs in addition to blog IDs

It is hard to determine what a blog's ID is.  This makes it hard for a user setting up LTI to determine what the proper endpoint for a blog should be.

A simple solution is to allow an LTI endpoint of the form /api/lti/blog-slug, rather than api/lti/blogid.  

This change adds that capability. This should not affect anything else.  Numeric blogids will still work.