Currently multiple consecutive spaces or tabs were being transformed into a single space, rendering some ASCII art unreadable.  This patch fixes it by giving each message text the CSS behaviour of <pre/>.

The `_converse.session` store gets cleared after logout, but we want the
`trusted` flag to persist after logout.

Also update the documentation no that the `storage` config option has
been removed in favor of `trusted`.

So that it persists across page loads. Otherwise storage falls back to
the default, causing records to be in both local- and sessionStorage.

Additionally, update singleton models to have the 'id' available as a getter.

Otherwise multiple records gets stored in browserStorage, causing random
results being returned.

Also remove Converse from fullscreen background

Also fixes #1153

Fixes #1154