use jquery to focus on alert messages
making sequence titles available to non-sighted users
Removing selection styling from sass files

- removed ::selection css from reset.scss
- removed variables from variables.scss
- removed variable calls from course/base/base.scss and base/base.scss
added alt tags to course cover images and find course page logo
increased text contrast to make text more legible
increasing visibility of tab navigation on site by making focus state very obvious