accessibility: Add `aria-describedby` to login/forgot password (LMS-602)

Also adding the email description to the login page and applying the
same aria attribute to the correspondin input on that page.

From LMS-602:

> There is additional explanatory text, marked up in a span element after
> the email input. The input elements should be explicitly associated with
> this additional text using the aria-describedby attribute. This will
> require an ID to be assigned to the span element.
accessibility: Focus on error messages in Help modal (LMS-584)

> To make this change explicit to assistive technology, focus should be
> moved programmatically to this div after it has been made visible. To
> allow the div to be focused via scripting (without adding it to the
> regular focus order that keyboard users cycle through), a tabindex=”-1”
> should be added.

> In addition, the invalid fields should be given a an
> aria-invalid="true" attribute.
accessibility: Add aria-hidden attribute to the help modal (LMS-584)

Added a 'aria-hidden="true"' attribute to the modal initially, then
toggle the value of that attribute to "false" when the modal is shown.
Also remove the attribute when the modal is closed, whether it's through
a click on the close button or on the overlay.
accessibility: Replace close modal button <div> by <a> link (LMS-582)

The close button for the modal dialog was marked up as a paragraph,
containing a graphical text character, inside two div elements, with an
associated click event. This made the button unusable for keyboard
users.

Replaced <divs> with a link element (<a role="button"), adapted the CSS
accordingly, and updated the existing <a> modal close buttons.
accessibility: Manage keyboard focus on help modal dialog (LMS-581)

Modal dialogs should manage keyboard focus, ensuring that users can't
tab out of the modal dialog and back into the page itself (effectively
moving “behind” the dimmed modal). On the last focusable element of the
dialog, return focus back to the first focusable element.

Also move the close button to the start of the dialog's section, making
it the first focusable URL, to ensure focus matches the visual
representation of the dialog.
accessibility: Allow to close help modal with "escape" key (LMS-581)

Users should be able to close the modal with the "escape" key.
Add ARIA dialog role to Markdown editor dialogs
fix default for VERIFY_STUDENT in lms/envs/aws.py
Merge pull request #902 from antoviaque/accessibility

Help Modal - Accessibility Fixes
Make URL patterns to exclude from tracking configurable

* Add unit test coverage for this "exclusion from tracking" logic
* Exclude based on regex match instead of raw string comparison
* Exclude heartbeat requests from tracking
Merge pull request #966 from edx/jbau/fix/verify-student-aws-settings-defaults

fix default for VERIFY_STUDENT in lms/envs/aws.py
Merge pull request #950 from edx/gprice/dialog-role

Add ARIA dialog role to Markdown editor dialogs
Merge pull request #925 from mulby/fix/gabe/configurable-tracking-filter

Make URL patterns to exclude from tracking configurable