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

Some big updates were available, fixing compatibility with Django 1.11

Namespaces should be unique, but we include newsletters twice.
By overriding the namespace of the urlconfig we fix this problem.

Redirect board item in menu to point to current board

Closes #302

See merge request !324

Allow board events

See merge request !322

Add vacancies to partner sitemap

Closes #289

See merge request !320

Add events sitemap to site sitemap

Closes #288

See merge request !319

Update and fix translations for events app

Closes #292

See merge request !318

Make sure models are ordered in admin dropdowns

Closes #277

See merge request !314