is_available method needed

[jace]

Both the requires_permission and the forthcoming requires_roles decorators (in ModelView; see #179) should provide an is_available method that returns True if a call to the decorated function will be allowed by the decorator under current conditions. Templates can use this to determine whether to show functionality to the user.

Currently templates do their own testing using obj.current_roles.role or current_auth.permissions.perm. This is risky because it can go wrong in either direction. Templates may incorrectly assume that the user has access to some functionality, or worse, templates may assume no access while the backend does in fact provide access, leading to security vulnerabilities. If templates use the same testing mechanism as the backend, we get closer to WYSIWYG UI, making it easier to spot misconfiguration.

[jace]

Closed in #219.