API: Make subscription API more RESTfuL

What does this MR do?

Only use POST requests to subscribe and unsubscribe.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/28327

spec/requests/api/v3/labels_spec.rb

@@ -71,7 +71,7 @@
   describe "POST /projects/:id/labels/:label_id/subscription" do
     context "when label_id is a label title" do
       it "subscribes to the label" do
-        post api("/projects/#{project.id}/labels/#{label1.title}/subscription", user)
+        post v3_api("/projects/#{project.id}/labels/#{label1.title}/subscription", user)
 
         expect(response).to have_http_status(201)
         expect(json_response["name"]).to eq(label1.title)
@@ -81,7 +81,7 @@
 
     context "when label_id is a label ID" do
       it "subscribes to the label" do
-        post api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
+        post v3_api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
 
         expect(response).to have_http_status(201)
         expect(json_response["name"]).to eq(label1.title)
@@ -93,7 +93,7 @@
       before { label1.subscribe(user, project) }
 
       it "returns 304" do
-        post api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
+        post v3_api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
 
         expect(response).to have_http_status(304)
       end
@@ -101,7 +101,7 @@
 
     context "when label ID is not found" do
       it "returns 404 error" do
-        post api("/projects/#{project.id}/labels/1234/subscription", user)
+        post v3_api("/projects/#{project.id}/labels/1234/subscription", user)
 
         expect(response).to have_http_status(404)
       end
@@ -113,7 +113,7 @@
 
     context "when label_id is a label title" do
       it "unsubscribes from the label" do
-        delete api("/projects/#{project.id}/labels/#{label1.title}/subscription", user)
+        delete v3_api("/projects/#{project.id}/labels/#{label1.title}/subscription", user)
 
         expect(response).to have_http_status(200)
         expect(json_response["name"]).to eq(label1.title)
@@ -123,7 +123,7 @@
 
     context "when label_id is a label ID" do
       it "unsubscribes from the label" do
-        delete api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
+        delete v3_api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
 
         expect(response).to have_http_status(200)
         expect(json_response["name"]).to eq(label1.title)
@@ -135,7 +135,7 @@
       before { label1.unsubscribe(user, project) }
 
       it "returns 304" do
-        delete api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
+        delete v3_api("/projects/#{project.id}/labels/#{label1.id}/subscription", user)
 
         expect(response).to have_http_status(304)
       end
@@ -143,7 +143,7 @@
 
     context "when label ID is not found" do
       it "returns 404 error" do
-        delete api("/projects/#{project.id}/labels/1234/subscription", user)
+        delete v3_api("/projects/#{project.id}/labels/1234/subscription", user)
 
         expect(response).to have_http_status(404)
       end