Post menu: separate 'sensitive' from 'spoiler text'.

Post menu: separate 'sensitive' from 'spoiler text'.

Alas, it _is_ possible to tag a post as sensitive but not provide any
spoiler text. I don't think I should make it actually impossible
through the UI of this client, so here's a change to permit it.

But I'm discouraging it as much as possible, by focusing into the
content-warning editor as soon as you mark a post as sensitive. That
way, if you really want an absence of warning, you have to
deliberately refuse to type one and press Return.