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.
MDL-40854 fix mod/...:view capabilities

Prior to the fix, if you did not have a capability like mod/page:view,
then you woulds still see the link to the Page activity in the course
section, but when you clicked on it, you would run into a
require_capability error.

It is a principle that we never show users a link to a page they are not
allowed to access, therefore, when users do not have mod/...:view, they
should not see the link on the course page.

This patch implements this in the cm_info class, in a similar way to how
access restrictions by groups works.

It does not assume that the mod/...:view capability exists. If the
capability does not exist, then users are not prevented from seeing the
link.