LMS Problem-centric Accessibility Tweaks

This pull request contains revisions to LMS problem templates that focus on accessibility, including:

* **show answer button text** - the text/content for this button has been revised to be clearer to users using assistive tech (includes a necessary HTML element change, references to new HTML element in coffeescript/js, and revisions to Sass/styling for new HTML element)
* **individual problem check status** - where appropriate, a problem's status displays have been ARIA associated with the main input element for the problem
* **individual problem input + answer association** - where appropriate, a problem's main input element(s) have been ARIA associated with the provided answer content

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Some larger questions for problem technical and content architects came up because of this:

* a lot of the changes to multiple files in this PR were identical, should they be abstracted out to avoid redundancy/manage maintenance
* do we need to account for grader status with ARIA roles?
* I've applied the necessary ARIA tags for element with a class of "status" or "grader-status" (in the cases where immediate checking isn't available) within problem templates, does this cover everything with respect to the current status of student's checking/receiving feedback on their responses to a problem?