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

Remove duplicate CSS and unused images from theme

See merge request !329

Change 'jaarlaag' to 'cohort'

See merge request !339

Enforce committee membership enddates in the past

Closes #284

See merge request !335

This also fixes the is_staff constraint, as memberships
that ended today should not give staff access anymore. This is
a particularly important case, as this is typically what happens
when memberships end 'today' and the object is saved.

Merge branch '303-commissieleden-die-lid-zijn-sinds-hebben-commissielid-sinds-1970-bij-hun-avatar-staan' into 'master'

Show ? for missing committee join dates

Closes #303

See merge request !334

Add pip conflict checks to testing pipeline

See merge request !333

Update requirements

See merge request !327

Update introduction text for vacancies page

Closes #247

See merge request !332

Improve board urls

See merge request !325

Add descending ordering to thabloids

See merge request !336

Fix order events in list view

Closes #304

See merge request !331

Enable TinyMCE clean on text pasting

Closes #237

See merge request !328

Add link to gitlab in footer

Closes #299

See merge request !317

Fix years in statistics

Closes #297

See merge request !330

Now uses the current lecture year instead of the current calendar year.

Validations should be a list

See merge request !323

Fix uniqueness of url namespaces

See merge request !326

This breaks Django2 otherwise