fix(email-notif): ability to unsubscribe from notification emails (#186)

[adrienjoly]

Closes #186.

• fixes reduction of frequency for email notifications
• fixes message after unsubscribing from notification emails
• refactors unsubscribe logic, for better clarity
• adds tests for email notifications
• adds browser.clickOnVisibleSelector() wdio command
• adds webUI.signup() for tests

[adrienjoly]

$ npm run docker-seed && wdio wdio.conf.js --spec test/specs/email-notifications.tests.js

[chrome #0-0] reduce frequency of email notifications
[chrome #0-0]   ✓ create a test user
[chrome #0-0]   ✓ user has instant email notifications"
[chrome #0-0]   ✓ user activates daily email notifications
[chrome #0-0]   ✓ user now has daily email notifications"
[chrome #0-0]   ✓ user reduces frequency of notifications from email
[chrome #0-0]   1) user sees switch to weekly notifications emails
[chrome #0-0]   2) user now has weekly email notifications"
[chrome #0-0]   ✓ user logs out
[chrome #0-0]
[chrome #0-0] unsubscribe from email notifications
[chrome #0-0]   ✓ create a test user
[chrome #0-0]   ✓ user has instant email notifications"
[chrome #0-0]   ✓ user unsubscribes from email
[chrome #0-0]   3) user sees unsubscription message
[chrome #0-0]   ✓ user was effectively unsubscribed from email notifications"
[chrome #0-0]   ✓ user logs out
[chrome #0-0]
[chrome #0-0]
[chrome #0-0] 11 passing (38s)
[chrome #0-0] 3 failing

1) reduce frequency of email notifications user sees switch to weekly notifications emails:
response should contain "weekly"

2) reduce frequency of email notifications user now has weekly email notifications":
-1 == 7

3) unsubscribe from email notifications user sees unsubscription message:
response should contain "never"