ProviderAccount fixes for rails 3.2.11

Call load_provider on ProviderAccount edit:

@provider is used in edit.html.haml but isn't loaded.  This worked
with rails 3.2.3 but stopped working with rails >= 3.2.4.  I have no
idea how it ever worked originally.  This explicitly calls
load_provider, but only after calling require_privilege (and thus not
from before_filter).  Otherwise load_provider will throw an exception
if the correct permissions aren't set.

Add load_provider to before_filter for update:

In update, if provider_account doesn't validate, the view renders the
edit action, which needs @provider in order to construct the edit
path.