Add a development setup

[tomasr8]

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Depends on #28

Adds a new containerized development setup which is similar to the production one.

Extra features:

• Pulls from master
• Includes maildump
• Creates a default admin user to skip the manual setup
• Adds an announcement saying not to use this setup in production
• Pulls translations from Transifex (either using a public mirror or directly when given an API token)
• Alpine base image to cut down the size (still more than 1GB)

[ThiefMaster]

• We probably shouldn't use uwsgi for this but the dev server (to get the reloader)
• "Indico will be accessible at localhost:8080. You can also access the wsgi app directly at localhost:9090 which skips the nginx proxy" is something best to get rid of since w/ base url enforcement it won't be possible to access indico via different URLs at the same time, and it's generally a bad idea anyway

[tomasr8]

Thanks for the comments @alexiri! I'll push some updates. (I also rebased the PR on the latest master)