This is a fix for https://github.com/conversejs/converse.js/issues/1508

when creating attributes for outgoing message.

Otherwise `id` is the same for all messages.

Until now, we've only been explicitly documenting the `_converse.api`
namespace and only considered the methods under it as forming the API
contract (which determines how we do semver releases).

It appears as if we've reached a point where trying to keep everything
under the `_converse.api` namespace no longer makes sense. Certain
methods are applicable to particular models and trying to shoehorn them
into the `_converse.api` namespace seems clunky and non-intuitive.

I've therefore decided to slightly refactor `sendMessage` to let it take
two simple parameters and to document it with JSDoc so that it's
presented as an API method, albeit only available on a chat model.

updates #1496
updates #1504

fixes #1352

updates #1352

updates #1352

Eventually we need to add better support for receipts and markers of MAM
messages.

We'll need to do the following:

* First check whether the MAM page already contains the receipts or markers, to avoid duplication
* Only ever in catchup mode
* Only after full catchup to ensure that the receipt is not in a different page than the message

Currently, in order for Converse.js to recognize a pasted URL as an
image, it must be an URL whose protocol is `https:` (`https` in the
URI.js library's notation). This is sensible, but means that any
non-HTTPS image URL is not recognized as a valid URL (and thus will not
be rendered inline, even if `show_images_inline` is set to `true`).

It is important to always check for HTTPS URLs when in a secure context
(i.e., the initial page load was requested via HTTPS) in order to ensure
that non-secured content does not mix with secured content. However, the
inverse is not true: if the original page was loaded over HTTP, then
enforcing HTTPS for images adds arguably no meaningful protection while
also breaking the `show_images_inline` feature for the edge cases where
Converse.js is deployed without HTTPS and a user pastes an HTTP URL.

This patch changes the behavior of the `isImageURL` method such that the
requirement for the pasted URL's protocol to be `https:` is enforced
only when the `window.location.protocol` itself is also `https:`. By
doing this, we ensure that secure origins (i.e., when Converse.js is
loaded over HTTPS initially) are still secured and cannot have non-HTTPS
content introduced to the page via a pasted non-HTTPS URL, however it
also allows non-HTTPS origins to render both HTTP and HTTPS image URLs.

Before the function `extractReference` in
`/src/headless/converse-muc.js` matched url and email.

Fix: https://github.com/conversejs/converse.js/issues/1327

With MAM2 we can just use stanza-id

This helps us to avoid duplicates by relying on the fact that Backbone
models have unique ids.

Update Lithuanian (lt) translation

As per XEP-0004, the default "type" of data form fields is
"text-single", so a missing "type" attribute should not be treated
differently.

This fixes handling of CAPTCHAs offered by ejabberd.

Removes the need for maintaining a map of keycodes to keys

* Accepts other triggers besides tab or @
* More finegrained control over whether the triggering char is
  included in the returned value or not.