* Add Backend Logic for Toggling Email Notification

This commit adds the backend logic for
allowing users to enable or disable email
notifications. The implementation ensures
that only issue notification emails get disabled
and important emails are still sent regardless
of the setting.

The UI to toggle this setting has not yet been
implemented.

* Add UI and complete user email notification enable

This commit completes the functionality to allow
users to disable their own email notifications.

Signed-off-by: Gary Kim <gary@garykim.dev>

* Add Third Option for Only Email on Mention

Signed-off-by: Gary Kim <gary@garykim.dev>

* Readd NOT NULL to new preference string

Signed-off-by: Gary Kim <gary@garykim.dev>

* Add Tests and Rewrite Comment

Signed-off-by: Gary Kim <gary@garykim.dev>

* Allow admin to set default email frequency

Signed-off-by: Gary Kim <gary@garykim.dev>

* Add new config option to docs

Signed-off-by: Gary Kim <gary@garykim.dev>

* Fix a few mistakes

Signed-off-by: Gary Kim <gary@garykim.dev>

* Only update required columns

Signed-off-by: Gary Kim <gary@garykim.dev>

* Simplify an error check

Signed-off-by: Gary Kim <gary@garykim.dev>

* Make email_notification_preference column in DB be VARCHAR(20)

Signed-off-by: Gary Kim <gary@garykim.dev>

* Handle errors

Signed-off-by: Gary Kim <gary@garykim.dev>

* Update models/migrations/v93.go

Co-Authored-By: Lauris BH <lauris@nix.lv>