Use shared form partial for admin/announcements views

[mjankowski]

Both of these views had repeated markup to generate the form. Changes:

• Pull out the common form tags to a new partial - the only difference here was the check for published? in the edit view. This is preserved in the form partial, and since published? will always be false on the new view, it works there too.
• In both views, switch the inner form variable from f to form which lets us do a nicer shorthand render form for the partial (auto-chooses file name and assigns correct local var)
• In both views, instead of doing the explicit path/method setup on the form, use the array/namespace option to simple_for and generate it that way. This puts the correct http method in the form as well.