Thumbnailing refactor

Closes #378 and #395

See merge request !1155

Add timestamp signature validation for non-authenticated users

Document thumbnailing

Add signing snippet for full image urls

Always sign private image urls

Sign thumbnail urls with url path

Always sign generate thumbnail urls and use data from signature

Remove django-sendfile

Include README files in source in documentation using m2r

Rename info variable to sig_info

Remove SECRET_KEY from production.py

Closes #818

See merge request !1146

Set postgres version in docker-compose.yml

See merge request !1151

Merge branch '478-add-booleanfield-that-toggles-public-display-of-membergroup-members' into 'master'

Add 'Display members' field for MemberGroup

Closes #478

See merge request !1150

Use Python 3.7 for Docker builds

See merge request !1148

Replace 'supporter' value in Membership model by 'benefactor'

Closes #712

See merge request !1141

Add country field to user profiles

Closes #810

See merge request !1140

Add wire payment type to payments app

Closes #813

See merge request !1145

Events payments refactor

See merge request !1112

Fix all entityType values of conscribosync

See merge request !1142

Don't cascade user deletion into deletion of exams etc

See merge request !1144

Unify used tablesorter plugins

Closes #804

See merge request !1143

Set download attribute on Thabloid download links

See merge request !1138

Remove miscellaneous from top of documents page

See merge request !1139

Set right entity type in conscribo sync

See merge request !1137

Conscribo member sync

Closes #790

See merge request !1132

Fix events unpublish

See merge request !1135

Automatic pizza save fix

Closes #807

See merge request !1130

Add translation for hardcoded "Locatie"

See merge request !1136