Update Docker env config for Events network

[iandunn]

#907 added some basic config for the Docker env, but more is needed to fully support and provision the new Events network.

• Basic wp-config.php updates
• Basic nginx.conf updates
• Add nginx rule for /xmlrpc.php, something like rewrite "/[\w-]+/\d{4}/[\w-]+/xmlrpc\.php" /xmlrpc.php last;
• Add nginx rule for uploaded files, something like rewrite "/[\w-]+/\d{4}/[\w-]+/files/(.+)$" /wp-includes/ms-files.php?file=$1 last;
• Create new root site and example event sites. Update EVENTS_ROOT_BLOG_ID if it doesn't already match.
• Comprehensive testing

Other changes may be needed as well.

[iandunn]

I merged #912, which added the parts that I needed to do in order to work on #907. More changes and testing will be needed to make this issue complete, though.

[iandunn]

I moved the task for updating the database to #668, because that's related and it's messy to try and have competing diffs for the SQL export

[timiwahalahti]

I tested the Events network, and it seems to work as expected. I will leave this open, in case someone else wants to do more testing.

[StevenDufresne]

I'll close this for now, it has been working for me and @timiwahalahti also gave it a test.

We can reopen if we come up with a clearer definition of "Comprehensive testing", otherwise things could just be logged as bugs in the issue tracker as normal.