Invitation decline confirmation

Targets #1391 (comment 122246)

Summary

1. Removes the feature of causing the invitation to be declined as soon as the decline invitation page is opened (or clicked from email)
2. Instead the user will have to confirm the decline action by clicking on a "Decline invitation" button
3. Updated wording on the page and the thank you message after taking the action

Questions to clarify

1. Reviewer invitation and Author invitation links both redirect to same page that says "confirm to decline invitation for peer review". Not sure if this is correct wording for author invitation.
2. The previous decline action (when opening the page) rejects the invitation without any value in response_comment and declined_reason. Now, as the functionality requires submitting button, we are saving either "OTHER" or "DO_NOT_CONFIRM" in declined_reason based on checkbox check/uncheck.