Adds UI fields for previously hardcoded e-mail templates
Added note to section header telling users which shortcodes are available
Refreshing POT with updated strings
Updating shortcode placeholders in e-mail template strings

Translation placeholders were used for shortcodes inside templates when the templates were hardcoded, but now the actual shortcode slugs should be used because the templates are stored in the database.
Fixed misplaced translation placeholder
Merge branch 'email_templates'. Adds UI for previously hard-coded templates.