Clickable links to Achievements and Societies

Closes #828

See merge request !1160

Fixed Visual Error Committees

Closes #791

See merge request !1149

Fix pizza order where pizza event has no notification, closes #819

Closes #819

See merge request !1163

fix for api assuming non external registration

Closes #822

See merge request !1162

Changed Date Format in List View + Translations

Closes #823

See merge request !1159

Merge branch 'bug/committee-links' of gitlab.science.ru.nl:thalia/concrexit into bug/committee-links

Update pipfile.lock

See merge request !1161

Add membership type to payment notes

See merge request !1147

Fix vars in fstr in conscribosync.py

See merge request !1152

Fix registration payment object migration values

See merge request !1153

Add more functionality to payments admin (search, export, fields)

Closes #815

See merge request !1154

Do not show cohort 2019 yet on the members overview page

Closes #824

See merge request !1156

Improve Sphinx generated documentation

Closes #826

See merge request !1158

Merge branch 'bug/committee-links' of gitlab.science.ru.nl:thalia/concrexit into bug/committee-links

Further clarify that technical change is for the Technicie

See merge request !1157

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