by not importing Strophe outside of @converse/headless

Mesage versions modal didn't open on subsequent clicks
Autocomplete: avoid undefined error

And remove it as a plugin.

As demanded by XEP-0393

And when nested, have no spaces between them.

Rename converse-core.js to core.js

Allows 3rd party plugins to add more buttons to the message actions dropdown

Set passed-in properties on modal

* Calculate proper length for :geo URI matches
* Don't strip out line-break after quotes, instead use CSS `display: inline-block`

Otherwise if an endpoint was passed in with `converse.initialize`, then
that endpoint's protocol is (wrongly) used.

to determine valid characters to form a boundary before an `@` mention

Also fixed an issue with mentions looking like they're part of URLs, by
first processing mentions separately.