Validate tkey in /post/discussion
Apply forced formatting to editdiscussion too
Disallow blank Format and fallback to Html
Revert "Apply forced formatting to editdiscussion too"

This reverts commit 38539e5097f794a9d7b2a6d1f0e30091911e7ceb.
No special case for blank FormatMethod
Block Format change to discussion when InputFormatter is forced
Merge pull request #2356 from vanilla/hotfix/postdiscussion

Security fixes to how discussions are posted and edited