Fix marking everyone at event as present

Closes #323

See merge request !354

Make sure the send_welcome_email field is not required

Closes #327

See merge request !358

Use lecture year for documents

Closes #328

See merge request !359

Make sure absolute urls are used in newsletters

Closes #315

See merge request !353

Fix problem with late cancellation when events has no 'max participants' number

See merge request !355

Make code coverage build-specific

See merge request !357

Fix inverted NL<>EN for translated vacancy text

Closes #247

See merge request !356

Make announcements work using production settings

See merge request !351

The newsletter migration can't be run atomically

See merge request !346

Betere default sorteringen voor pizzas in orders en overview

Closes #136

See merge request !344

Added support for uploading albums as tar files

Closes #263

See merge request !341

Fix 'option 3' text in styleguide

See merge request !343

Make documentation great again

See merge request !342

test for errors or warnings

Archives other than .zip and .tar.gz are rejected, even though the
tarfile library supports files with lzma and bzip2 compression, as the
MIME types for these compressions are not registered and the values in
request.content_type are user supplied, so these may vary. Perhaps we
should use libmagic in the future to avoid these issues.

Support voor het uploaden van albums als tar files.
Ik heb geprobeerd om code duplicatie zoveel mogelijk te vermijden, maar
de inconsistenties tussen zipfile en tarfile vermoeilijkten dit.
De (oude) code onnodig een mapje aan voor het nieuwe album, maar Django
doet dit al automatisch dus ik heb die code verwijderd.

Update session-stored partners upon changes

Closes #159

See merge request !337

Also now does it with a list comprehension as the tuple just
consumes the generator anyway.

Create announcements

Closes #250

See merge request !338

Fix comparison of board in 'validate_unique'

See merge request !340