Made newly added feedback requests show immediately on sending

This is achieved by including a part of the HTML layout
in the JSON response. This is what the template breakdown
into smaller pieces has been initially done for.
Fixed the feedback error behaviour and style

This is done to make the 'no content' error more obvious,
since it will not disappear until a user enters something
in the text box.
Made the custom text get properly treated when sending more requests.

Before that the placeholder flag was not being reset to false,
which resulted in sending of messages with placeholders instead
of real values.
Replaced strings with constants as they were intended.