Add a template filter for message placeholders

As a first guess placeholders can be added to messages with the
`((placeholder))` syntax.

This commit adds a Jinja template filter to convert strings containing
said-formatted strings into HTML, which can then be styled to highlight which
parts will be substituted in messages.

app/assets/stylesheets/main.scss

@@ -25,9 +25,9 @@
 @import "govuk_elements/elements/panels";
 @import "govuk_elements/elements/tables";
 
-// ////---xxsd----k/
 // Extra CSS for this application
 @import "components/sms-message";
-
+@import "components/placeholder";
+// // // sxx
 // Break this up
 @import "app";