Merge branch '470-login-with-local-django-accounts-and-ldap-password-changes-are-mutually-exclusive' into 'master'

Resolve "Login with local Django accounts and LDAP password changes are mutually exclusive"

Closes #470

See merge request !847

This is only set if LDAP is enabled on the site being migrated, to
ensure the change in 42f5708d does not
leave existing sites vulnerable to users authenticating with shadow
copies created before the change.

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