Site HTML: make links relative

Due to Lerna's hoisting, it's in the top-level node_modules

* Remove CleanWebpackPlugin
    This pugin would wipe the non-minified files every time `make dist` was called,
    and generally made it more difficult to build only particular files.
* Use order-only prerequisites for the `dist` Make recipe
    This allows more efficient building because order-only prerequisites
    don't force a rebuild of the main recipe.
    https://www.gnu.org/software/make/manual/make.html#Prerequisite-Types

This fixes anonymous logins

For OMEMO in MUC, use the real JID of the user, not the MUC JID

Bumps [lodash](https://github.com/lodash/lodash) from 4.17.15 to 4.17.19.
- [Release notes](https://github.com/lodash/lodash/releases)
- [Commits](https://github.com/lodash/lodash/compare/4.17.15...4.17.19)
Signed-off-by: dependabot[bot] <support@github.com>

instead of querying the stanza.

Also fixes a bug where typing notifications triggered an AttributeError
inside `shouldNotifyOfGroupMessage`

Fixes a bug whereby carbons weren't being enabled because the listener
for `afterResourceBinding` was registered before `converse.initialize`
was called,(which means it was removed as soon as `cleanup` was called
inside `converse.initialize`).

This shows that event `api.listen` API calls can/should only be called
inside the `initialize` method of a plugin (or at least once plugins
have been initialized).

Removed Jed from the headless build and instead let `__` call sprintf.

It's now up to downstream users for `@converse/headless` to decide
whether they want i18n support for the few translatable strings in that package
and to implement it themselves.

This makes it possible to wait for the connection_status before trying
to call `getOwnOccupant` in plugins

Precursor to some refactoring planned

- Add hooks to the stanza parsers so that plugins can do additional parsing.
- Change ChatBox instance methods to functions and use them for stanza parsing.
- Move encrypt and decrypt messages to `converse.env.omemo`

Apparently, when receving a 1:1 carbon message, a device was wrongly created
for the contact's device list, instead of our own.

So that `_converse.xmppstatus` is defined before trying to set the
status.

before `converse.initialize` has been called

In the process, remove other "core" promises. There is no core or
community plugins code awaiting on them and since it's possible to
call `converse.initialize` twice, I'm not sure it makes sense to even have them.

Also removed unused error class IllegalMessage.