Added methods for detecting MySQL/PostgreSQL

These two methods remove the need for manually going into
ActiveRecord::Base.connection all over the place.

lib/gitlab/database.rb

+module Gitlab
+  module Database
+    def self.mysql?
+      ActiveRecord::Base.connection.adapter_name.downcase == 'mysql'
+    end
+
+    def self.postgresql?
+      ActiveRecord::Base.connection.adapter_name.downcase == 'postgresql'
+    end
+  end
+end

spec/lib/gitlab/database_spec.rb

+require 'spec_helper'
+
+describe Gitlab::Database do
+  # These are just simple smoke tests to check if the methods work (regardless
+  # of what they may return).
+  describe '.mysql?' do
+    subject { described_class.mysql? }
+
+    it { is_expected.to satisfy { |val| val == true || val == false } }
+  end
+
+  describe '.postgresql?' do
+    subject { described_class.postgresql? }
+
+    it { is_expected.to satisfy { |val| val == true || val == false } }
+  end
+end