Disable data URLs in top-level navigation

[jumde]

Description:

Data URLs should not be allowed in top-level navigation

Steps to Reproduce

1. Navigate to jumde.github.io/test/data1.html
2. Click Hello

Actual result:
Should not navigate to a data URL

Expected result:
Navigates to a data URL

Reproduces how often: [Easily reproduced, Intermittent Issue]
Easily

[jumde]

https://blog.mozilla.org/security/2017/11/27/blocking-top-level-navigations-data-urls-firefox-59/

[jumde]

We'll be using the same exceptions specified in the blog.

[iccub]

Bumping to 1.16 for now since this requires more work and thought, if we will be able to fit it into 1.15.1 I will change the milestone back