IMHO selecting in how many headings it's possible to support investments
isn't necessary when there's only one option to choose from. It's
obvious that if there's only one heading, it will be impossible to
select investments from more than one heading.
We usually prefer local variables over instance variables in partials.
This way we'll be able to call the partial from views or components
where the instance variable isn't available.
And since we're using the `path` variable to configure the URL, we don't
have to specify extra variables like `@budget` or the namespace `:admin`
in `form_for`, since Rails only uses those variables to set the URL.
This way we can simplify the way we generate form fields. In some cases,
we also use the human attribute in table headers, which IMHO makes
sense.
I haven't moved all of them: for example, sometimes a label is
different depending on whether it's shown to administrators, valuators,
or users. And I haven't touched the ones related to devise, since I
wasn't sure about possible side effects.
Note I've also removed placeholders when they had the same text as their
labels, since they weren't helpful. On the contrary, the added redundant
text to the form, potentially distracting users.
To make it more consistent with the rest of the Admin panel,
the CRUD for budget groups and headings has been changed
from the old "all-in-one" form to a separate form for each resource.
