Jsa/fix new post cat selector

@gwprice 
* supersedes https://github.com/edx/edx-platform/commit/a13863f29d5a80bbda1ee558f6eab66ae49deba4 and https://github.com/edx/edx-platform/commit/127d1c5c20470892750c0505be6e37b8173a4c82
* incorporates suggestion to enhance cat text filtering behavior