service settings views: simple typing fixes

[risicle]

And some ignore markers where we're reaching in to add extra errors to form fields, which we should probably find better ways to do.

This PR makes one important decision - allowing assert statements in production code, but only for the purposes of type-narrowing. In these cases it's to tell the type checker something it can't figure out itself - that a valid form won't have None values for certain fields. These are all cases where a None being present would have thrown an exception anyway because of what the following code was trying to do with it.

This seems preferable to me than wrapping each use of these .data attributes in cast(...) calls, which would also have not given certainty about correctness in the way assert statements do.

[risicle]

Oops sorry @CrystalPea, was trying to click @quis for review (this is a strawman for something we were already discussing).. but take a look if you like anyway.