[Sprint/OwlfashionedOwl](fix): Limit hashtag remind #1927

Closes #1927 (closed)

Summary

Adds an error if a user tries to submit more than 5 hashtags when reminding a post

Testing

Pick a post, remind, type in "#1 (closed) #2 #3 (closed) #4 (closed) #5 (closed) #6" or something to that effect.