Set SECUREDROP_ENV for development environment

This makes running certain tasks manually, such as the individual
`source.py` or `journalist.py` applications, less error-prone. You need
to set `SECUREDROP_ENV=dev` in order to have `FlaskConfig.DEBUG = True`,
which is necessary for Flask to serve the static files.
Merge pull request #649 from freedomofpress/set-securedrop-env-for-development

Set SECUREDROP_ENV for development environment