Rename the action operation types related with template linkage

The discovery and autoregistration actions currently have inconsistent names of the operation types regarding the template linkage: "Link to a template" and "Unlink from a template". This names looks confusing, because in UI hosts configuration we are able to link a template to a host, not backwards. To prevent the possible confusings the relation between template and host always should be presented in one way, as "Template is linked to host or unlinked from host". Such presentation of the relation between host and template should be used in all places of UI, API, Documentation.