[Frontend] Better sorting indicators added to styles

[charlesh88]

https://github.jpl.nasa.gov/MissionControl/vista/issues/48
Simplified styles to indicate sort by asc and desc in table th elements;
(cherry picked from commit 15a2416)

Author checklist:

• Changes address original issue? Y
• Unit tests included and/or updated with changes? N/A
• Command line build passes? N/A
• Expect to pass code review? Y

Assumes that we'll add ng-click to all relevant th elements of the table. Add the following to the class attribute of the th of any table that's sortable:

• Add "sort asc" or "sort desc" (depending on sort direction) when that column is controlling the sort order.
• Add "sortable" to the th of any column that's clickable to control sort order. This displays the pointer cursor when the user hovers over the th. Don't add this to columns that can't control sorting.

Screenshot added of what the interface should look like in a VISTA datatable, sorting asc by "scet".

[larkin]

Review Checklist

Changes appear to address issue? Y
Appropriate unit tests included? (N/A - css only)
Code style and in-line documentation are appropriate? Y
Commit messages meet standards? Y
Project-specific information isolated to appropriate branches? Y

Integration Checklist

Automated build passing after merge? Y
No merge conflicts (or resolution trivial/obvious)? Y
All master branches up-to-date after merge? Y
Project-specific information isolated to appropriate branches? Y