A `Status` has a `spoiler_text` property and a `sensitive` property.

On the web UI Mastodon, if you create a `Status` with spoiler text the
`sensitive` property is set to `true`. This is front end behaviour,
not something enforced by the API.

The previous code was assuming that `sensitive` being `true` also meant
there was spoiler text, and vice-versa, but this is not the case, all
four possible options are possible (empty/non-empty spoiler text x
true/false sensitive property).

This meant that a preview card on a `Status` with `spoiler_text` but
not marked `sensitive` would still be displayed.

Re-write the code to control whether the `Status` body is displayed to
use the `spoiler_text` property exclusively.

The `sensitive` property is still used to determine how to display
images in preview cards.

Fixes pachli#2280