Still lacks inter-tab communication to update state across tabs, i.e.
when sending a 1-on-1 message in one tab, it doesn't appear in another,
because that information is not available via the websocket connection.

- Create a new `Connection` class that extends Strophe.Connection and
    move related code from `converse-core.js` into this class.
- Store the session in localStorage when using a worker
- Move XEP-0156 code to connection.js
    This allows us to initialize the connection without needing to know the
    domain.

* fixes 2059 add a separator between views

If a message comes in after joining the MUC but before the MAM messages
have been fetched, then converse-mam will query for MAM messages newer
than that message, causing an empty history (except for that one message).

It's currently only used by the MUC header to render URLs in the subject

- Don't replace entire textarea when entering a second emoji
- Focus emoji picker on tab completion when it's already opened

for emoji related data

Fixes #2126

(i.e. not on reconnection)

the first time it's opened

Changes how rich message markup is generated and kept track of.
We now have a MessageText class which subclasses String and which keeps
track of the rich markup associated with the message.