MDL-40857 External tool (LTI) capabilitiy names confusing.

I have improved the names in the language file so that they actually say
what they do (based on my reading of the code). I have also added
comments to access.php explaining each capability.

I have corrected a few of the RISK in access.php which were wrong.

I have changed a couple of the archetypes so that guests cannot launch
LTI activities, and editing teachers cannot do editing things.
MDL-40857 External tool: fix incorrect moodle_url usage.