This fixes #470, where local Django accoutns were generally locked if
LDAP accoutns were used together with password handling to protect
against deleted/locked LDAP users being able to still login using a
shadow copy of their account in the Django database.

The fix introduces user account attributes, and the LDAP
authentication code keeps a record of users who used to authenticate
with LDAP in the past. If a suer is known to have been using LDAP in
the past, they are denied if they cannot be authenticated in the
future; if a user tries to authenticate who has not used LDAP in the
past, they are allowed in.

Resolve "AccountSignupForm accesses database on instantiation"

Closes #586

See merge request !846

Release 2.4

See merge request !842

Resolve "Login timeouts if ldapserver is unavailable"

Closes #582

See merge request !844

User invitations

Closes #453

See merge request !450

[CI] Prepare lock file on build

See merge request !843

Removeresponsive tables for now

See merge request !838

Update dependency django-colorfield to ^0.6.0

See merge request !841

Fix ordering of changelog after merge accident

See merge request !840

Resolve "Date picker for date of birth unusable"

Closes #569

See merge request !835

Resolve "Favicon should contain only school bag"

Closes #254

See merge request !772

# Conflicts:
#   CHANGELOG.rst

Resolve "Icons/logo should be uploaded to public/"

Closes #581

See merge request !837

# Conflicts:
#   CHANGELOG.rst

Resolve ""Imprint" is called "imprint""

Closes #579

See merge request !834