Custom field values of type tag not persisted correctly

[KennethWussmann]

Currently, selected tags of a custom field with type TAG are saved to the value column in the DB.
They should be saved in a new Many-to-Many table to allow selecting multiple tags without loosing the link between field values and tags.

The search index of assets should be flattened to something like location: office, kitchen if the user selected two tags for a custom field.

[KennethWussmann]

Persistence part is done, but the TagSearchInput doesn't support it now.