Ensure good comprehension hygiene using flake8-comprehensions.

* Remove redundant python2 support code

`str.decode()` doesn't exist on python3, so presumably this code was doing
nothing

* Filter out pushers with corrupt data

When we get a row with unparsable json, drop the row, rather than returning a
row with null `data`, which will then cause an explosion later on.

* Improve logging when we can't start a pusher

Log the ID to help us understand the problem

* Make email pusher setup more robust

We know we'll have a `data` member, since that comes from the database. What we
*don't* know is if that is a dict, and if that has a `brand` member, and if
that member is a string.

The `http_proxy` and `HTTPS_PROXY` env vars can be set to a `host[:port]` value which should point to a proxy.

The address of the proxy should be excluded from IP blacklists such as the `url_preview_ip_range_blacklist`.

The proxy will then be used for
 * push
 * url previews
 * phone-home stats
 * recaptcha validation
 * CAS auth validation

It will *not* be used for:
 * Application Services
 * Identity servers
 * Outbound federation
 * In worker configurations, connections from workers to masters

Fixes #4198.

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

It doesn't really belong under rest/client/v1 any more.

The Mailer expects the config object to have `email_smtp_pass` and
`email_riot_base_url` attributes (and it won't by default, because the default
config impl doesn't set any of the attributes unless email_enable_notifs is
set).