Allow specifying the tenant in budget tasks

The `budgets📧selected` and `budgets📧unselected` tasks are
supposed to be run manually because they only make sense at a specific
point during the life of a budget.

However, they would only run on the default tenant, and it was
impossible to run them on a different tenant.

So we're introducing an argument in the rake task accepting the name of
the tenant whose users we want to send emails to.

lib/tasks/budgets.rake

@@ -1,13 +1,13 @@
 namespace :budgets do
   namespace :email do
     desc "Sends emails to authors of selected investments"
-    task selected: :environment do
-      Budget.current.email_selected
+    task :selected, [:tenant] => :environment do |_, args|
+      Tenant.switch(args[:tenant]) { Budget.current.email_selected }
     end
 
     desc "Sends emails to authors of unselected investments"
-    task unselected: :environment do
-      Budget.current.email_unselected
+    task :unselected, [:tenant] => :environment do |_, args|
+      Tenant.switch(args[:tenant]) { Budget.current.email_unselected }
     end
   end
 end