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/lib/validators/timezone_validator_spec.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe TimezoneValidator do
+  describe "#valid?" do
+    context "when timezone is ok" do
+      it "returns true" do
+        expect(described_class.valid?("Australia/Brisbane")).to eq(true)
+      end
+    end
+
+    context "when timezone is not ok" do
+      it "returns false" do
+        expect(described_class.valid?("Mars")).to eq(false)
+      end
+    end
+  end
+
+  describe "#validate_each" do
+    let(:record) { Fabricate(:active_user).user_option }
+
+    context "when timezone is ok" do
+      it "adds no errors to the record" do
+        record.timezone = "Australia/Melbourne"
+        record.save
+        expect(record.errors.full_messages.empty?).to eq(true)
+      end
+    end
+
+    context "when timezone is blank" do
+      it "adds no errors to the record" do
+        record.timezone = nil
+        record.save
+        expect(record.errors.full_messages.empty?).to eq(true)
+      end
+    end
+
+    context "when timezone is not ok" do
+      it "adds errors to the record" do
+        record.timezone = "Mars"
+        record.save
+        expect(record.errors.full_messages).to include(
+          "Timezone 'Mars' is not a valid timezone"
+        )
+      end
+    end
+  end
+end