Unify setupmethod behavior between Flask and Blueprint

[davidism]

Many of the methods in the Flask app object are decorated with setupmethod, which prevents registering things with the application after it starts running. A similar behavior is in Blueprint, we want to prevent modifying it after registering it at least once. However, the blueprint checking is much less consistent, it's divided up into two parts, is only a warning sometimes, and has a different message. Figure out how we can make Flask and Blueprint more consistent with this error.

[hallacy]

@davidism I can start on this one

[davidism]

I also noticed that setting app.debug, ConfigAttribute, and app.config keys, could also reasonably be considered setup. I haven't applied a warning or error to these yet, but it's something to think about. It's a little more complicated because technically config should be finished as soon as the extension that uses it (or the app) is initialized. There are also other things, such as template config that shouldn't change after app.jinja_env is accessed, or static URL config after the static route is added.