The legend html was not escaped and was ignoring block settings coming
from the form editor.
This commit adds a new helper function for rendering legend that respects
applies block settings and also escapes the output.
[MAILPOET-4471]
Added a 'type-message' for Parsley to display an error message when the input field is filled in incorrectly.
Also removed the global Parsley message to prevent it from overruling the 'required-message'.
[MAILPOET-4471]
screen-reader-text CSS class only exists within the WordPress environment
the class does not exist when using iFrame forms due to these being used outside the WordPress context
prefixing with mailpoet-* to not interfere with the default WordPress screen-reader-text class
MAILPOET-4312
