Improve docs for the announcements module

Closes #588

See merge request !724

Clean up and document utils package

Closes #587

See merge request !726

Fix other test warnings

See merge request !728

Suppress unclosed file handle warnings

Closes #589

See merge request !727

Fix event registration saved Dutch message

See merge request !729

One about timezones, the other about a deprecationwarning

Further improve documentation

Include docs for utils submodules

docs build needs the dev requirements as it will import commands

Further improve docs

Delete unused ci-requirements.txt

Closes #571

See merge request !725

Improve docs for the merchandise site

See merge request !723

Closes #571

Also makes the view that handles marking announcements as closed much more robust

See #570

See #570

Update available tox environments in README

See merge request !722

Add more tests to the events app

See merge request !717

Refactored documents app

Closes #268 and #519

See merge request !689

Install binary version of psycopg2

See merge request !720

Get manage.py from Django 2.0

See merge request !719

Use app configs

See merge request !721

This is recommended by the Django documentation

Move Payment model to dedicated 'payments' app

See merge request !704

Surpresses a warning and we haven't had issues with the binary package

See also http://initd.org/psycopg/docs/install.html#binary-install-from-pypi

This strips comments about Python 2 and uses Python 3 raise from exception code.

Fix coverage badge in README

See merge request !718

Fix availability of photo albums on the website

Closes #566

See merge request !714