Correct issue where list-style bullets are seen on BP forms on message screens

[hnla]

In some browsers and on certain screens bullets may be seen caused by twentyfifteen stylesheet ruleset overriding BP rules.

Add a further property to standard form and group form to remove them.

[hnla]

In 10319:

Correct a bullet point issue on bp forms where twentyfifteen styles might override BP list-style: none.
Adds a further list-style none to standard and group forms child li elements.

See #6689 Props slaffik