Also, remove ephemeral messages after 10 seconds, not 20

Use-case is when an invalid MUC gets opened in singleton mode.
Without this fix a new chat with spinner appears, even though it
shouldn't.

updates #555

- Remove the `keepalive` configuration setting. It is now always implicitly `true`.
- Remove the `expose_rid_and_sid` configuration setting.
- A `prebind_url` is now mandatory when setting `authentication` to `prebind`.
- It's no longer possible to pass in `rid` and `sid` values to `converse.initialize.

I'm using prettierx, which is a fork of prettier so that we can have
spaces before the brackets in a function declaration.

I don't like how restrictive prettier is concerning its configuration
and formatting.

- it messes up some indentation in specs/
- it removes a second newline when I specifically want to have two
- it doesn't allow spaces before brackets in function declarations (hence prettierx)
- it adds spaces for function parameters with default values

None of this is configurable.

So auto-formatting is not turned on by default, but it might be a useful
tool in some cases.

Fixes an issue where indentation was off in converse-core.js, causing
vim-sleuth to wrongly indent by three spaces only.

- fix import path
- remove unnecessary lines in webpack config
- remove unused file

updates #1111

* Checkmark and edit modal button are now on the right
* Instead of showing checkmarks for MUC messages,
  we now show them as light grey before MUC reflection
* Simplify message markup by requiring less `if` statements

Also, set the resource in `setUserJID`

- New plugin `converse-smacks`
- New config option `enable_smacks`
- Rename session cache id from `converse.bosh-session` to `converse.session`
- Refactor logout and login as consistently used api methods
- Refactor session cache to store per JID

Fixes #316

So that we can specify defaults

When a synchronous event is fired, Converse will wait for all
promises returned by the event's handlers to finish before continuing.

if the page is reloaded while the nickname form is shown