Allow free-text reasons for "inappropriate" flagging

Right now, if someone flags a post as “inappropriate,” they’re not able to explain why they thought it was inappropriate in the flag submission box. Then, it’s up to the moderators to figure out (guess) why the flagger considered the post inappropriate. The user can type a reason only if they select “something else” as the flag reason.

It would be useful to allow users to explain why they thought a post was inappropriate. (We can always follow up via DM, but that’s more work.)