rewrite flagging fields admin help text to make use of 'provides form' link type property

#1734494: add property to hook_flag_link_type_info() to let link types declare they provide a form has added a property to flag link types to let them declare that they show the user a flagging entity form.

Therefore, the help text on the 'manage fields' page can be rewritten to programmatically list the link types that the admin user can enable to allow the flagging fields to be used.

Furthermore, it can warn when a link type that doesn't provide a form is currently in use. (Follow-on: add this as a warning when the flag form is saved with a non-form link type and fields exist on the flagging.)

Current help text reads, in part:

You will also want to pick either the "Confirmation form" or the "Form" link type for your flag, or else users won't have a means to enter values for the fields. (In case a form isn't used, the fields are assigned their default values.) Note: You don't have the Flagging Form module enabled. You'll have to enable it to have the "Form" link-type.

It's probably a good idea to keep the link to the Flagging Form module project though, if it's not present, so users can easily discover it.

(Tagging as Novice, but it's probably more Novice-ish...)