A11Y changes: better :focus styling, increased contrast, and added image alt texts

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y
- Changed video caption styling to blue and made them underline on hover and added a skip link before video for screenreaders.

Fixes Bugs:
- LMS-1336

common/lib/xmodule/xmodule/css/video/display.scss

@@ -595,7 +595,7 @@ div.video {
 
     li {
       border: 0;
-      color: #666;
+      color: rgb(29,157,217);
       cursor: pointer;
       margin-bottom: 8px;
       padding: 0;
@@ -613,7 +613,7 @@ div.video {
       }
 
       &:hover {
-        color: $blue;
+        text-decoration: underline;
       }
 
       &:empty {

common/lib/xmodule/xmodule/templates/about/overview.yaml

@@ -19,7 +19,7 @@ data: |
       <h2>Course Staff</h2>
       <article class="teacher">
         <div class="teacher-image">
-          <img src="/static/images/pl-faculty.png" align="left" style="margin:0 20 px 0">
+          <img src="/static/images/pl-faculty.png" align="left" style="margin:0 20 px 0" alt="Course Staff Image #1">
         </div>
 
         <h3>Staff Member #1</h3>
@@ -28,7 +28,7 @@ data: |
 
       <article class="teacher">
         <div class="teacher-image">
-          <img src="/static/images/pl-faculty.png" align="left" style="margin:0 20 px 0">
+          <img src="/static/images/pl-faculty.png" align="left" style="margin:0 20 px 0" alt="Course Staff Image #2">
         </div>
 
         <h3>Staff Member #2</h3>