Fixed #18134 -- BoundField.label_tag now includes the form's label_suffix

There was an inconsistency between how the label_tag for forms were
generated depending on which method was used: as_p, as_ul and as_table
contained code to append the label_suffix where as label_tag called on a
form field directly did NOT append the label_suffix. The code for
appending the label_suffix has been moved in to the label_tag code of
the field and the HTML generation code for as_p, as_ul and as_table now
calls this code as well.

This is a backwards incompatible change because users who have added the
label_suffix manually in their templates may now get double label_suffix
characters in their forms.