FEATURE: Add timezone to core user_options (#8380)

* Add timezone to user_options table

* Also migrate existing timezone values from UserCustomField,
  which is where the discourse-calendar plugin is storing them

* Allow user to change their core timezone from Profile

* Auto guess & set timezone on login & invite accept & signup

* Serialize user_options.timezone for group members. this is so discourse-group-timezones can access the core user timezone, as it is being removed in discourse-calendar.

* Annotate user_option with timezone

* Validate timezone values

spec/requests/session_controller_spec.rb

@@ -1048,6 +1048,16 @@ RSpec.describe SessionController do
           expect(user.user_auth_tokens.count).to eq(1)
           expect(UserAuthToken.hash_token(cookies[:_t])).to eq(user.user_auth_tokens.first.auth_token)
         end
+
+        context "when timezone param is provided" do
+          it "sets the user_option timezone for the user" do
+            post "/session.json", params: {
+              login: user.username, password: 'myawesomepassword', timezone: "Australia/Melbourne"
+            }
+            expect(response.status).to eq(200)
+            expect(user.reload.user_option.timezone).to eq("Australia/Melbourne")
+          end
+        end
       end
 
       context 'when user has 2-factor logins' do