Overhaul content warnings

Summary

Fixes #1137 (closed)

Overhauls the display of CWs.

Basically allows marking a whole status as sensitive and replaces the old "show more" design of CWs with the same clickthrough warning used for sensitive media.

This simplifies both features.

• Remove the "Mark media as sensitive" checkbox from attachments in the post composer.
• When the spoiler button is active, always send sensitive: true on submit.
• Move the spoiler input below the compose form (like with polls) and make it optional.
• Redesign statuses with spoiler text. Make them appear like regular sensitive posts, and include the spoiler text under "This content may not be suitable for all audiences".
• When replying to a post, don't inherit the sensitivity of the parent.

Screenshots (if appropriate):

Now clicking the ⚠ button marks the entire status as sensitive, even if there are no media attachments and even if spoiler text isn't entered.

When a post has a content warning, it's displayed behind a clickthrough warning.

Uploading a file no longer displays the sensitivity checkbox. To achieve the same thing, click the ⚠ button after uploading media instead.

Eg: