Add config setting `send_chat_state_notifications`

Hide "add contact" button and requesting/pending contacts when
allow_contact_requests is false.

We were removing the disco entity, but it's associated collections
(`features`, `fields`, `identities` etc) were still stored in
localStorage and not cleared.

So when the entity gets recreated, the stale localStorage cached items
repopulate the collections.

Added `refreshFeatures` API method for refetching disco features and use
that instead.

`toLocaleString` now returns element attributes in alphabetical order
(for better cross-browser consistency).

Also, `toLocaleString` is now used in favor of `outerHTML` because
browsers aren't consistent with one another in their output.

Otherwise compatibility with Conversations is broken.
Unfortunately this means that OMEMO won't work on Edge, because it
for some reason requires the additionalData.

References:
* https://github.com/conversejs/converse.js/issues/1187#issuecomment-426276243
* https://stackoverflow.com/questions/42933162/web-crypto-api-using-microsoft-edge-38-14393-0-0/42950731

Fixes #1187

Currently translated at 99.7% (338 of 339 strings)

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

Currently translated at 65.4% (222 of 339 strings)

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

Currently translated at 25.6% (87 of 339 strings)

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

Currently translated at 12.0% (41 of 339 strings)

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

Currently translated at 97.6% (331 of 339 strings)

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

Currently translated at 82.5% (280 of 339 strings)

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

Currently translated at 84.6% (287 of 339 strings)

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

Also udpate markup to not use `<source>`.

updates #1189