UI Enhancement: Add support for displaying and creating content warnings.

[rabble]

Some scuttlebutt clients support adding a field 'contentWarning' in the posts which then are displayed instead of the full content of the post. That field is free text and the user is encouraged to write their own text.

It would be good to support displaying content warnings and let users create contentWarning

[martindsq]

needs design from Chardot.

First add support for displaying them.

[martindsq]

part of swiftui post redesign #744

[rabble]

The ISIS and Furry content shows us we need this. Also it's part of our terms of service but we don't let users do it.