Accessibility: Account-based Template Follow-up

This work finalizes the work on the following known issues:

* Log in page needs alt text (partially resolved already) - https://edx-wiki.atlassian.net/browse/LMS-600
* Field level descriptions not explicitly associated with fields (partially resolved already) - https://edx-wiki.atlassian.net/browse/LMS-1159
* Unnecessary use of fieldset element - https://edx-wiki.atlassian.net/browse/LMS-1157