This refactors the TodoService class according to our code reuse
guidelines. The resulting code is a wee bit more verbose, but it allows
us to decouple the column names from the input, resulting in fewer
changes being necessary when we change the schema.
One particular noteworthy line in TodoService is the following:
todos_ids = todos.update_state(state)
Technically this is a violation of the guidelines, because
`update_state` is a class method, which services are not supposed to use
(safe for a few allowed ones). I decided to keep this, since there is no
alternative. `update_state` doesn't produce a relation so it doesn't
belong in a Finder, and we can't move it to another Service either. As
such I opted to just use the method directly.
Cases like this may happen more frequently, at which point we should
update our documentation with some sort of recommendation. For now, I
want to refrain from doing so until we have a few more examples.
This refactors the TodosFinder finder according to the new code reuse
rules, as enforced by the CodeReuse cops. I also changed some of the
methods to use regular if statements, instead of assignments and/or
early returns. This results in a more natural flow when reading the
code, and it makes it harder to accidentally return the wrong result.
