[feat] Add unsubscribe link to email notifications

[Dhanus3133]

Fixes: #256

FYR, Please note that this feature depends on PR #290 to manage global email notification preferences, which is not implemented in this PR. Currently, the user's first notification will be treated as their global email notification preference.

Regarding the email token generated:

• No expiry.
• Invalidated once the user updates their password.

[nemesifier]

• please add tests
• please remove time based expiration
• if the user unsubscribes, we need to turn off all email notifications

[pandafy]

This feature need a lot of improvements in the current state. We shall work on the shortcomings and ensure the logic is robust as this will be a vital feature of the notifications module.

[pandafy]

I did a quick review of the code and found the following areas for improvement. I will do another round of functional testing and share my findings.

[nemesifier]

There's some conflicts here.