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?
Does this mean the user has created an account with edX but hasn't activated the account, or that she hasn't created an account at all? I'm guessing the former, but here are a couple of suggestions based on the answer.

-> You need to activate your {platform_name} account before you can register for courses. Check your inbox for an activation email.

-> You need to create an account on {platform_name} before you can register for courses. To create an account, (link)go to the edx.org registration page(link).