Fixes #1172

updates #497

When receiving a PreKeySignalMessage, then a prekey has been chosen and
should now be removed from the list of available prekeys in the bundle,
so that a different device doesn't choose it as well.

AFAICT, libsignal removes the prekey, so it's then up to us to
regenerate it and republish our bundle.

updates #497

Before these changes, prekeys were stored in two places, one place that
converse-omemo accessed and one that libsignal accessed and when
libsignal deleted a prekey the other store wasn't updated.

Now we let the methods called by libsignal store/remove prekeys (and the
signed_prekey) in the same place as used by the code in converse-omemo.

updates #497

updates #497

Updates #497

Updates #497

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/>.

updates #497

updates #497

Array Buffers can't be JSON serialized as is.

updates #497

updates #497

updates #497

updates #497

Instead of setting `active` to `false`, we remove the device entirely
(unless its the current device).

Doing it this way means more fetching of bundles for devices that
disappear and then reappear from a user's devicelist.

However, there might be caching invalidation concerns with just reusing
a cached bundle for a device id that disappeared and then reappears.

Additionally this change simplifies the showing of a contact's device
fingerprints in the modal, since we don't have to take active/inactive
into consideration.

updates #497

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`.