This reverts commit 7af73c3471.
Doesn't look like the right approach for adding support for XEP-0156.
Work on that will continue in a branch in the meantime.
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.
