Adds d6 and d7 migrations for audience, membership and roles

Commits from GitHub:

• Alter the default membership type bundle name in field migration rows
• Fix some typos in migration plugins and improved migration module info
• Use the source plugin id instead of the migration id in hook_migrate_prepare_row
• Replace two migration ids with their corresponding source plugin ids in og_migrate_migrate_prepare_row
• Fixed comments in hook_migrate_prepare_row
• Fix the OgFieldD7MigrateTest kernel test
• Add dependencies in the OgFieldD7MigrateTest kernel test
• Skip migrating the og_membership_request field on the default group membership type
• Fix a typo in og_migrate_migrate_prepare_row
• Fix and improve handling default membership type in og_migrate_migrate_prepare_row
• Fix a couple of coding standard issues and add assetions for the form and view displays of the default membership type

Closes #3468826

og_migrate/migrations/d6_og_admin_role.yml

+id: d6_og_admin_role
+label: Og admin roles
+migration_tags:
+  - 'Drupal 6'
+  - Configuration
+source:
+  plugin: d6_og_admin_role
+  constants:
+    entity_type: node
+process:
+  id: id
+  label:
+    plugin: default_value
+    default_value: 'administrator'
+  weight:
+    plugin: default_value
+    default_value: 0
+  is_admin: is_admin
+  group_id:
+    plugin: default_value
+    default_value: 0
+  group_type: constants/entity_type
+  group_bundle:
+    -
+      plugin: migration_lookup
+      migration: d6_node_type
+      source: type
+    -
+      plugin: skip_on_empty
+      method: row
+      message: 'Missing content type'
+destination:
+  plugin: entity:og_role
+migration_dependencies:
+  required:
+    - d6_og_group_type