Improve copy on opt-out page for learners

[dirkcuys]

This current opt out page looks like this:

Opting out for a learner means to stop participating in a learning circle. The current screen gives almost no context and doesn't communicate to the user that it implicates ending participation.

Maybe

• Improve copy - change from "Opt Out. Please confirm that you wish to leave this learning circle." to something more accurate
• Include more context about the learning circle
• Explain the results to be expected from the action - no more reminders, won't be able to complete the survey, facilitator won't be able to contact them using p2pu tools, ...

note the opt-out page is unauthenticated. the URL uses an UUID for the learner and a signature, but the URL could be inadvertently shared with someone else, so we should make sure that any context we include is appropriate and won't be leaking any sensitive data to unauthenticated users.