Add process to review suggestions

This is the simplest I could make review process for adding new suggestions.

Basically, this will be the flow:
1. A user adding a suggestion will have it created with is_reviewed false.
2. An admin will then occasionally go over the suggestions while marking items reviewed as such.
3. The admin may also filter items that have yet to be reviewed and also mark multiple items as spam, using the helpful "mark as spam" custom admin action.

* bonus: every suggestion that is saved with a state other than new will automatically be marked as reviewed, thus removing the need to explicitly mark items as reviewed.