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

lib/validators/timezone_validator.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class TimezoneValidator < ActiveModel::EachValidator
+  def self.valid?(value)
+    ok = ActiveSupport::TimeZone[value].present?
+    Rails.logger.warn("Invalid timezone '#{value}' detected!") if !ok
+    ok
+  end
+
+  def self.error_message(value)
+    I18n.t("errors.messages.invalid_timezone", tz: value)
+  end
+
+  def validate_each(record, attribute, value)
+    return if value.blank? || TimezoneValidator.valid?(value)
+    record.errors.add(
+      attribute,
+      :timezone,
+      message: TimezoneValidator.error_message(value)
+    )
+  end
+end