Validate that host hostname is lowercase

Currently the site importer (`lib/transition/import/site_yaml_file.rb`)
downcases any hostnames.

This adds validation to the host model which will be surfaced on the new site
form.

app/models/host.rb

@@ -12,6 +12,7 @@ class Host < ApplicationRecord
   validates :hostname, hostname: true
   validates :site, presence: true
   validate :canonical_host_id_xor_aka_present, if: -> { hostname.present? }
+  validate :hostname_is_downcased, if: -> { hostname.present? }
 
   after_update :update_hits_relations, if: :saved_change_to_site_id?
 
@@ -71,4 +72,10 @@ def update_hits_relations
     hits.update_all(mapping_id: nil)
     Transition::Import::HitsMappingsRelations.refresh!(site)
   end
+
+  def hostname_is_downcased
+    if hostname != hostname.downcase
+      errors.add(:hostname, "must be lowercase")
+    end
+  end
 end

spec/models/host_spec.rb

@@ -75,6 +75,18 @@
           expect(host.errors_on(:hostname)).to include("is an invalid hostname")
         end
       end
+
+      context "is downcased" do
+        subject(:host) { build :host, hostname: "AB.com" }
+
+        describe "#valid?" do
+          subject { super().valid? }
+          it { is_expected.to be_falsey }
+        end
+        it "should have an error for invalid hostname" do
+          expect(host.errors_on(:hostname)).to include("must be lowercase")
+        end
+      end
     end
   end