Also adds a shallow Service model that coexists with the config-defined
services to group multiple OAuth2 and API clients together.

Clients defined in the config with OAUTH2_CLIENTS and API_CLIENTS_2 are
imported by the database migrations.

Removes support for complex values for the OAuth2 client group_required option.
Only simple group names are supported, not (nested) lists of groups previously
interpreted as AND/OR conjunctions. Also removes support for the login_message
parameter of OAuth2 clients.

Previously User used salted SHA512 with OpenLDAP-style prefix syntax and
Signup used crypt. Both models had their own hashing and verification
code. Now both use OpenLDAP-style syntax with support for all traditional
formats including crypt. Salted SHA512 is used for new User and Signup
passwords.

Existing Signup objects are migrated to the new format and remain functional.
User passwords now support gradual migration to another hash algorithm when
it is changed in the future.

This code is planned to be used for database-stored API and OAuth2 client
secrets.

* Removal of ldapalchemy and LDAP mocking support
* Removal of dependency on ldap3 (except for the migration)
* Remaining "LDAP_<name>" config keys are renamed to "<name>"
* Web interface to create, edit and delete groups
* Consistent foreign key, cascading and nullable configuration on all models
* User/Group.dn is replaced with numeric User/Group.id
* User.uid is renamed to User.unix_uid (to differentiate with new User.id)
* Group.gid is renamed to Group.unix_gid (to differentiate with new Group.id)
* All User/Group/Mail related routes now use the database ids instead of uid/gid/dn
* PasswordToken/MailToken now reference users directly instead of storing loginnames

The database migration (optionally) uses the v1 config keys to connect to
an LDAP server and to import all users, groups and mail forwardings.

Fixes #104.

Replaced "group" keyword argument for login_required with "permission_check".
Most views already define a *_acl_check function that returns whether the
current user has the required permissions for use with register_navbar. The
same function can now be passed to login_required as the "permission_check"
argument.

Differenciated login and selfservice access permissions. Previously
ACL_SELFSERVICE_GROUP was required to login. Now ACL_ACCESS_GROUP is required
to login and ACL_SELFSERVICE_GROUP is required to access selfservice functions
(and to use role-granting invite links). A user with just ACL_ACCESS_GROUP can
now login, access the services overview page and authenticate with OAuth2
services he has access to, but not change his user attributes, password or
roles/permissions.

Ldap3 raises LDAPSASLPrepError on bind if the password contains characters
forbidden by SASLPrep (string preperation/normalization algorithm for user
names and passwords). Examples are carriage return ("\r") or newline ("\n")
characters. See #100.

For password reset this prevents circumventing the loginname/email-based
ratelimit.

Add a few unit tests as well as integration tests that cover almost all views. Also fixed a lot of HTML validator errors.