[CAPT-1862] Add unsubscribe functionality

[asmega]

Context

• https://dfedigital.atlassian.net/browse/CAPT-1862
• Add unsubscribe link to reminder emails
• NOTE: notify templates must to be updated AFTER this change has been deployed

Changes

• Send unsubscribe_url as part of reminder emails in the personalisation section so they can be appended to emails sent to users
• Should a user click on this link they are sent to the send to confirm if they wish to unsubscribe
• As recommended by notify https://docs.notifications.service.gov.uk/ruby.html#unsubscribe-link-at-the-bottom-of-the-email-recommended
• Upon success they get show the confirmation panel that transaction is complete
• Should user visit invalid link or if they are already unsubscribed they are show a page with copy explaining the situation
• config.action_controller.action_on_unpermitted_parameters has been changed back to the rails default setting which happens to also be the setting used in production. not sure why this was ever changed in the first place as it makes development and test behave differently to how production would behave

Notes

• One click unsubscribe is not present in this change, however the server side mechanism has been added
• This is because the library we are using at the moment does not support this notify feature see Support unsubscribe links dxw/mail-notify#166
• We either need to switch to official library or add the needed functionality to support this feature
• The last commit points to a modified gem of mail-notify which includes a change to support one click unsubscribe. Please remove last commit if one click unsub is not required

Screenshots

After user click unsubscribe link in email

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Upon successfully unsubscribing

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Wrong link or user already unsubscribed

[asmega]

resource_class is not currently used. but has been added so can be extended to support unsubscribing of other resource in the future easily if needed

[asmega]

• empty scope is used here so form does not use nested parameters
• this means the structure of the data supports a simpler format allowing the same controller endpoint to support one click unsubscriptions

[slorek]

Thanks for implmenting soft delete - could I ask that you instead re-use the Deletable concern to avoid duplication