domain 8.x-1.0-alpha6

There are some major revisions in this release.

If you update the module, you must rebuild the container (drush cr).

Major issues fixed:

* Issue #2820932: Fatal error when deleting a field by hostdog: Fatal error when deleting a field
* #2815951: When a user edits a entity/node that's shared across multiple domains, it ends up referencing only their domain(s) reported by FooZee and bunty_badgujar. Adds user form handling to hidden fields. This change allows the use of node and user forms by editors without access to all domains.
* Patch by @lulbina. Fixes url aliases in path lookups.
Implements a DomainElementManagerInterface.
* From @zerolab: Domain config overrides can now be set in settings.php.
* From @zerolab. Updates and optimizes Travis configuration.

For developers:

We have introduced a new service class, the DomainElementManager. Its purpose is to handle the following use-case:

* An editor with limited access to domains edits an entity that is assigned to domains outside their permission range.

Before this release, edits of this sort deleted the domains outside the users scope. The new service provides a standard means of handling this case.

For most field cases, the base manager works fine -- see domain_access_form_node_form_alter(). For special handling, extend the class in a new service, as shown in domain_source_form_node_form_alter().