Add ./dev bash script for dev actions

[rafalp]

This PR adds ./dev bash script that provides single point of entry for running dev commands such as creating dev project, clearing it, making messages, running tests, etc. ect.

Fixes #1079

Some cool stuff:

Running ./dev init will cause following to happen:

• if port 8000 is already taken, we tell user to shut down whatever is running on it and try again
• if port 5432 is already taken, we tell user to stop system-running PostgreSQL instance and try again
• if there are remaining files from old ./dev init present, we kindly ask user to run ./dev clear and try again
• we install everything needed to run misago, create the devproject with required dirs, expose manage.py, migrate the DB, create super, and run dev server
• we display short welcome message to user telling them to go

COMMANDS

• Initialize new dev project
• Delete existing dev project
• Rebuild docker
• Make messages
• Compile messages
• Pull from transifex
• Push to transifex
• Shortcut for running all tests
• Shortcut for running tests in module
• Expose psql
• Shortcut for docker-compose run --rm misago
• Shortcut for bash
• Shortcut for manage.py

EXTRA

• In some commands check if they are ran in docker before continuing
• Display some help text in default run
• Display guide for next step after ./dev init completes
• Add help for connecting with datebase
• Add command for getting post-init intro after already initializing the dev project
• Update readme
• Remove deprecated bash scripts
• Remove extras directory

[rafalp]

CC @einarf could you find a minute and give this work a look? There's still some work to be done here, but features look like they are done.

This is also sort of test run for proper appctl script that I plan to write for misago-docker down the road.

[einarf]

At first glance this looks very reasonable. You have at least the features I would like to see.