Merge feature SOL-20

This is a pull request to merge the SOL-20 feature (https://openedx.atlassian.net/browse/SOL-20).
The fact that this is sortable is not apparent to a screen reader user.   There is also no support for keyboard access here.   Suggest:
```<span class="column-sort-link" id="js-asset-name-col" role="button" tabindex="0"><%= gettext("Name") %> <%= gettext("Sortable") %></span>```
This will give the clickable text a role of button, make it keyboard focusable (tabindex) and add the screen reader only text "Sortable"