Add an option to unsubscribe the user from email notifications

[bubafinder]

I'm submitting a...

• Bug report
• Regression (a behavior that used to work and stopped working in a new release)
• Feature request
• Documentation issue or request

Context

Add an option on the settings page to unsubscribe the user from email notifications related to the repository.
The option will unsubscribe the user from:

• new comments
• assignments

[hrvojevu]

Part 1 of this issue in #892

[rkusan00]

Transferred → tailor-cms/author#517