Handle manually removed resources
Closes #209 (closed) #278 (closed)
Also fixes the testAccCheckGitlabGroupMembershipDestroy acceptance test that would always succeed previously given it would check the non-existing gitlab_group_membership.accessLevel resource attribute (instead of "access_level").
Also adds importers (and related documentation) for the following resources:
gitlab_tag_protectiongitlab_pipeline_schedulegitlab_pipeline_schedule_variable
Also fixes gitlab_tag_protection attribute access_level read persistence (uncatched type conversion error).