Improve Canva Conferences Invites

[jfederico]

Summary:

The current implementation of Canvas Conferences does not allow filtering on the invite list: it selects either all or none. For schools which have many observers, for example, it makes it difficult to use Conferences as the observers will be automatically added or manually deleted.

Steps to reproduce:

Expected behavior:

Additional notes:

This enhancement can be a workaround for https://community.canvaslms.com/ideas/11618-security-issue-observer-permissions-in-conferences-needs-to-match-canvas