Support LTI Advantage Custom Parameter Substitution

Go into an LTI tool in admin and add two custom items: zippy=$User.id zappy=$Context.title This tool can be 1.1 or 1.3, direct launch or Content Item / Direct Link.   All combinations should work.  Turn on debug to make it easier to see the launch data. Place the tool in Lessons and launch it - and look at launch data.   You should see the custom values but with the "$" expressions unchanged. With the fix, you will see the values substituted.  LTI 1.3 puts custom values in a JWT claim and LTI 1.1 puts them in post parameters - just search for zappy or zippy in the debug output.