Cleanup reserved words in generateSlug

closes #1889
- Adds a new filter for slugs


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Things to consider:

1. The filter allows apps/other places to easily extend/remove items from the list. What should the filter be called?
2. Is there a better place for the list of reserved slugs?
3. Which keywords should be removable and which should be fixed? 

Might want to add a comment here describing what these two arrays are used for.