Fix acessibility in content module links and timed exams

[sandroscosta]

This PR solves some issues with the link styles on the module. The link color was not sufficient for color blind users. As for timed exams, the start button was incorrectly formated.

The two issues were fixed:

• Place a text underline on links that are created in the content area.
• Fix the styling of the start button on timed exams.

The timed exam start button is now displayed correctly:

Related with https://github.com/fccn/edx-platform/issues/16 https://github.com/fccn/nau-themes/issues/46

[igobranco]

Related to this change: Place a text underline on links that are created in the content area Before:

After: