[TASK] Allow comma separated template marker form sg_forms to display meaningful labels in the all variable. Other bugfixes

This commit adds proper error messages when mail sending via the
mail queue has failed. It also refactors the MailTemplateService that it
does not use duplicate code to send mails.

This commit changes the fetching of pages due to the
pages_language_overlay migrated into the pages table.

[BUGFIX] Fix writing of the form configurations if the folder is ḿissing, Fix default form extension key to project_theme