DEV: Improve API usage when creating * updating categories

The category model already has a default value for `color` and
`text_color` so they don't need to be required via the API. The ember UI
already requires that colors be selected.

The name of the category also doesn't need to be required when updating
the category either because we are already passing in the id for the
category we want to change.

These changes improve the api experience because you no longer have to
lookup the category name, color, or text color before updating a single
category attribute. When creating a category the name is still required.

https://meta.discourse.org/t/-/132424/2

spec/requests/categories_controller_spec.rb

@@ -120,16 +120,6 @@ describe CategoriesController do
         expect(response.status).to eq(400)
       end
 
-      it "raises an exception when the color is missing" do
-        post "/categories.json", params: { name: "hello", text_color: "fff" }
-        expect(response.status).to eq(400)
-      end
-
-      it "raises an exception when the text color is missing" do
-        post "/categories.json", params: { name: "hello", color: "ff0" }
-        expect(response.status).to eq(400)
-      end
-
       describe "failure" do
         it "returns errors on a duplicate category name" do
           category = Fabricate(:category, user: admin)
@@ -314,27 +304,6 @@ describe CategoriesController do
         expect(response).to be_forbidden
       end
 
-      it "requires a name" do
-        put "/categories/#{category.slug}.json", params: {
-          color: 'fff',
-          text_color: '0ff',
-        }
-        expect(response.status).to eq(400)
-      end
-
-      it "requires a color" do
-        put "/categories/#{category.slug}.json", params: {
-          name: 'asdf',
-          text_color: '0ff',
-        }
-        expect(response.status).to eq(400)
-      end
-
-      it "requires a text color" do
-        put "/categories/#{category.slug}.json", params: { name: 'asdf', color: 'fff' }
-        expect(response.status).to eq(400)
-      end
-
       it "returns errors on a duplicate category name" do
         other_category = Fabricate(:category, name: "Other", user: admin)
         put "/categories/#{category.id}.json", params: {