Support LDAPSearchUnion now used in core

See merge request !13

Update dependency flake8-black to ^0.2.0

See merge request !15

Testsuite/Lint

See merge request !14

We expect it to be equal or all searches. If not, there is really
nothing that can be done for that LDAP admin.

Related to AlekSIS#249

Merge branch '17-no-such-preference-in-sitepreferenceregistry-with-section-ldap-and-name' into 'master'

Resolve "No such preference in SitePreferenceRegistry with section=ldap and name=additional_field_core_person_street_request"

Closes #17

See merge request !11

Resolve "Broken format strings in preferences"

Closes #18

See merge request !12

Currently translated at 100.0% (22 of 22 strings)

Translation: AlekSIS/AlekSIS-App-LDAP
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-ldap/de/

This reverts commit 1132fc11.

# Conflicts:
#	aleksis/apps/ldap/util/ldap_sync.py

Previously, the cache list was getting out of sync if some group produced an error.

Caught by @hansegucker.