Handle unavailable locales in subscriptions

There was an edge case where a user could configure a locale and then
the application would change the locales to that one would no longer be
available. In that case, we were getting a `I18n::InvalidLocale`
exception when accessing the subscriptions page.

So now, we're defaulting to `I18n.locale`. Note we're using
`I18n.locale`instead of `I18n.default_locale` because `set_user_locale`
is called inside the `switch_locale` block in `ApplicationController`,
which already sets `I18n.locale` based on `I18n.default_locale`.

app/controllers/subscriptions_controller.rb

@@ -32,8 +32,12 @@ class SubscriptionsController < ApplicationController
 
     def set_user_locale(&action)
       if params[:locale].blank?
-        session[:locale] = I18n.available_locales.find { |locale| locale == @user.locale&.to_sym }.to_s
+        session[:locale] = find_locale.to_s
       end
       I18n.with_locale(session[:locale], &action)
     end
+
+    def find_locale
+      I18n.available_locales.find { |locale| locale == @user.locale&.to_sym } || I18n.locale
+    end
 end

spec/controllers/subscriptions_controller_spec.rb

@@ -27,5 +27,13 @@ describe SubscriptionsController do
 
       expect(session[:locale]).to eq "es"
     end
+
+    it "only accepts available locales" do
+      create(:user, locale: "wl", subscriptions_token: "mytoken")
+
+      get :edit, params: { token: "mytoken" }
+
+      expect(session[:locale]).to eq "en"
+    end
   end
 end