Implemented confirm account via email feature

[amlannandy]

Description

PR consists of the following changes -

1. Whenever a new user is created, a confirmation email is sent to verify that account. Only after email confirmation, that account can use the routes protected by '@login_required'.
2. Modified the user model class to have an 'is_verified' property that is false by default.
3. Modified the '@login_required' decorator to also block requests by user with 'is_verified' = false.
4. Confirmation of the email sets 'is_verified' = true.

Fixes #140

Type of change

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Create a new user. Log in and try accessing protected routes. Then confirm the account and try again.

Test Configuration:

• Windows 10
• Google Chrome
• Postman

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules