#50798552 extend grace period

[Ashaba]

What does this PR do?

It enables users to extend grace period beyond 30 days

Description of Task to be completed?

Users are now able to choose grace period up to 60 days for on the check

How should this be manually tested?

1. Clone the repository
   git clone https://github.com/andela/healthchecks-invincibles.git
2. Checkout the branch
   git checkout ft-extend-grace-period-150798552
3. Activate the virtual environment and run the application
   ./manage.py runserver
4. Visit the application URL, http://127.0.0.0.1:8000 and log in
5. On the checks menu, click any checks available or create one and edit it's grace period

What are the relevant pivotal tracker stories?

#150798552

Screenshots (if appropriate)

[coveralls]

Coverage increased (+1.1%) to 81.353% when pulling 423426f on ft-extend-grace-period-150798552 into df137bf on master.

[coveralls]

Coverage remained the same at 81.353% when pulling 171ab44 on ft-extend-grace-period-150798552 into c5c0eaa on develop.

[coveralls]

Coverage remained the same at 81.353% when pulling 9e9abe2 on ft-extend-grace-period-150798552 into c5c0eaa on develop.