Add "cancel volunteering" button

[Mariatta]

Allow volunteers or team members to step down and cancel their participation in case they're no longer able to help out.

• Add the button to allow each volunteers to cancel
• Remove their membership from a team
• Set the status to cancelled
• Send email to the volunteer as confirmation that they've cancelled
• Don't show cancelled applications on the volunteer/list page.
• send email to the team lead with note that some offboarding tasks may be needed, eg removal of access, roles, etc.