Fix TypeError in call to bcrypt.hashpw

Fix some `/preview_url` explosions

- At the very least, this TypeError caused logins to fail on my own
  running instance of Synapse, and the simple (explicit) UTF-8
  conversion resolved login errors for me.

Signed-off-by: Salvatore LaMendola <salvatore.lamendola@gmail.com>

Enable testing the synchrotron on jenkins

Linearize fetching of gaps on incoming events

This potentially stops the server from doing multiple requests for the
same data.

Work around TLS bug in twisted

Correctly mark backfilled events as backfilled

Wrap up twisted's FileBodyProducer to work around
https://twistedmatrix.com/trac/ticket/8473. Hopefully this fixes
https://matrix.org/jira/browse/SYN-700.

Make get_domain_from_id throw SynapseError on invalid ID

Change /bin/bash to /bin/sh in tox.ini

No features of Bash are used here, so using /bin/sh makes it more portable to systems that don't have Bash natively (like BSD systems).

Add function to load config without generating it

Renames ``load_config`` to ``load_or_generate_config``
Adds a method called ``load_config`` that just loads the
config.

The main synapse.app.homeserver will continue to use
``load_or_generate_config`` to retain backwards compat.
However new worker processes can use ``load_config`` to
load the config avoiding some of the cruft needed to generate
the config.

As the new ``load_config`` method is expected to be used by new
configs it removes support for the legacy commandline overrides
that ``load_or_generate_config`` supports

502 on /thumbnail when can't contact remote server

Fix a bug caused by a change in auth_handler function

Remove redundant exception log in /events

Fix the relevant unit test cases

Pull full state for each room all at once

Don't make rooms visibile by default