Exclude "action tags" from topic list

[cjlee112]

We provide a list of optional tags (http://docs.selectedpapers.net/hashtags.html#optional-tags). If users include those in a post, selectedpapers.net indexes them as topic names, which doesn't make sense.

[ketch]

We actually don't support these tags as intended (they aren't bound to the paper). So perhaps they should be removed from the documentation, at least for now.