PostController: Add filter whitelist.

This is the start of an alternate fix for:  https://github.com/vanilla/vanilla/pull/1920

We will need to apply this filter to anything else that has input filter.  Activity and Activity comments are not complete yet and will need to be added for sure.

We will need to do regression testing on this to make sure it does not have any unwanted side effects.
Use sentences and always a space after the //.