view: prioritize Moodle capabilities over assumed access of hosts

[jrchamp]

I didn't know how to communicate my ideas clearly in #697. This pull request builds on that pull request, prioritizing Moodle capabilities over access assumptions for hosts and Zoom activity managers.

In summary:

• We don't need to check for a role switch, because the user is still the user.
• Meeting capacity warnings should be shown to all users who can edit the activity.
• If the user is a host (real or alternative), then they are already registered for the activity. A role switch does not and should not change their view of that.
• (optional) The "Start Meeting" message is too confusing. Hosts and non-hosts are both going to join the meeting. If join before host is enable, then the hosts aren't technically starting the meeting anyway, so simplify the logic and reduce the confusion.
• As we already have dedicated capabilities for viewjoinurl and viewdialin, it is wrong for us to override the Moodle administrator's choice of whether or not the user (based on their role) should have access to view that information. The Moodle administrator can decide whether viewjoinurl and viewdialin are granted to roles that can edit Zoom activities or not and we should respect/reflect that choice.

Net effect for complicated host variables:

• $userishost is now only used to determine if the user is pre-registered
• $userisrealhost does not emit PHP Notice when role switched

[smbader]

I really love the cleanup and move to a capability controlled logic as it should be within Moodle. Making this change inadvertently fixes the role switch problem now that we treat the viewing role as configured.