Resolve "Its possible to create personal events that start after their end"

Closes #960

See merge request !1703

Update dependency django-colorfield to ^0.12.0

See merge request !1705

Resolve "Use consistent setting names"

Closes #1092

See merge request !1577

Handle min & max in DateTimeField

See merge request !1704

Resolve "Make timezone field optional in personal event mutations"

Closes #1104

See merge request !1694

b64(a) + b64(b) ≠ b64(a + b) 

Evolution is a 

Resolve "Introduce non-dialog object form"

Closes #1110

See merge request !1655

Previously each use site would have to provide the logic. But should
better to do it only once!

Previous interface was (imoh) unused. The importance of the update and
the previously broken design warrent a break in interface.