MDL-45843 mod_lti: Fixes based on feedback

Minor changes and corrections based on review in JIRA
MDL-45843 mod_lti: Fixed bug in toolproxy service

Corrected bug which failed to respond properly to an invalid request
Also updated upgrade.txt file
MDL-45843 mod_lti: Improved admin navigation

Added the manage tool registrations page as a separate entry on the
admin menu (within a folder named LTI).  Made this entry the current
position for the related pages.