Clean up translated strings in registration page, dashboard, and track selection flow

As per [ECOM-288](https://openedx.atlassian.net/browse/ECOM-288), [ECOM-289](https://openedx.atlassian.net/browse/ECOM-289), and [ECOM-290](https://openedx.atlassian.net/browse/ECOM-290). I've done my best to clean up the strings displayed on these pages, making them consistent with the message and tone of the pages on which they appear, making them more clear when possible, and correcting any grammatical errors.

These changes will run into the same issue as the proposed string changes in #5114. Namely, these copy changes will break live translations of the lines I've touched, reverting them to English until Transifex translators can translate them again. If we need to merge these changes and @griffresch is aware of the impact, @sarina's recommendation is to merge this soon after a release candidate has been cut.

@wedaly and @stephensanchez, can you take a quick look at this?
And a comma splice! :-) 

-> We're sorry, but {platform name} isn't available in your region.
or
-> We're sorry. {platform_name} isn't available in your region.
or 
-> We're sorry. Unfortunately, {platform_name} isn't available in your region.