Update copy for security slider to be consistent with the mobile experience

We just finished our work with building the security slider experience for Orfox. We are very happy with the results we got from our user research on that project.

Since the slider on desktop now also has 3 options instead of 4. The UX team would like to suggest that we also use the same copy used for Orfox experience.

We hope to continue improve our user experience and this copy might change in the future as a result of that. In case of that happen we will make sure to keep the experience consistent on all platforms.

That new copy would be:

The results of our efforts! Both Orfox and Tor Browser have agreed to adopt these changes.

Standard 

All Tor Browser and website features are enabled. 

Safer

Disables website features that are often dangerous, causing some sites will lose functionality. 

At this cautious setting: 
* JavaScript is disabled on non-HTTPS sites.
* Some fonts and math symbols are disabled.
* Audio and video (HTML5 media) are tap-to-play. 

Learn more  (link to https://tb-manual.torproject.org/en-US/security-slider.html)

Safest

Only allows website features required for static sites and basic services. These changes affect images, media, and scripts. 

At the strict security setting: 
* JavaScript is disabled by default on all sites. 
* Some fonts, icons, math symbols and images are disabled.
* Audio and video (HTML5 media) are tap-to-play. 

Learn more  (link to https://tb-manual.torproject.org/en-US/security-slider.html)