Fixes scrollbar appearing next to spinner when logging in.

* Replace `tearDown` overrides with event handlers.
* Move presence listeners to `connected` event handler, otherwise they
  don't get called when logging in again after disconnection.
* Add `clearSession` listeners to clean up upon disconnection.
* Use `Promise.all` instead of listening for multiple events.

Currently translated at 100.0% (374 of 374 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/fr/

Currently translated at 57.0% (213 of 374 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/ru/

Currently translated at 64.7% (242 of 374 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/he/

Currently translated at 100.0% (374 of 374 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/cs/

Currently translated at 50.0% (187 of 374 strings)

Translation: Converse.js/Translations
Translate-URL: https://hosted.weblate.org/projects/conversejs/translations/eo/

Makes debugging more difficult and uses up extra CPU cycles for no real security benefit.

* Use font from variables
* No need to load bourbon twice

Also, store the returned `stanza-id` on the message.

Updates #324

This reverts commit 01f0a652.

Updates #1410

Don't check for older messages with same msgid
we can't rely on a message's `id` attribute being unique.

Also, remove `createMessage` in favour of calling `messages.create` directly.

updates #1405

Fixes #1405

Updates #1442