Apply Legislation Process default colors to dev seeds

Legislation Processes created through the admin form were getting the default color.
However, Legislation processes created by other means (like the `db:dev_seed` rake task) were not getting these default values.

This feature was originally implemented when we were using Rails 4.
With Rails 5, we can provide default values to all new Legislation processes
and simplify the code at the same time thanks to its `attribute` method.

Related commit:
https://github.com/consul/consul/pull/4080/commits/0b83be6

spec/models/legislation/process_spec.rb

@@ -10,6 +10,13 @@ describe Legislation::Process do
     expect(process).to be_valid
   end
 
+  it "assigns default values to new processes" do
+    process = Legislation::Process.new
+
+    expect(process.background_color).to be_present
+    expect(process.font_color).to be_present
+  end
+
   describe "dates validations" do
     it "is invalid if debate_start_date is present but debate_end_date is not" do
       process = build(:legislation_process, debate_start_date: Date.current, debate_end_date: "")