While people using screen readers already have keyboard shortcuts to
jump to the <main> tag, there are people who navigate the page with the
keyboard using just the tab key, and for them, this link provides a way
to save time and start reading the main content instead of having to
manually go through all the navigation links every time a new page is
loaded.
Note that we had to add an additional `width: 0` rule because
Foundation's `element-invisible` would apply `1px` and the test checking
for `visible: :hidden` would faile.
Many pages had this tag, but many other didn't, which made navigation
inconsistent for people using screen readers.
Note that there are slight changes in two pages:
* The homepage now includes the banner and the content of the
`shared/header` element inside the <main> tag
* The budgets index now includes the banner inside the <main> tag
I see both potential advantages and disadvantages of this approach,
since banners aren't necessarily related to the main content of a page
but on the other hand they aren't the same across pages and people using
screen readers might accidentally skip them if they jump to the <main>
tag.
So I'm choosing the option that is easier to implement.
Note we're adding a `public-content` class to the <main> element in the
application layout. This might be redundat because the element could
already be accessed through the `.public main` selector, but this is
consistent with the `admin-content` class used in the admin section, and
without it the <main> element would sometimes have an empty class
attribute and we'd have to use `if content_for?(:main_class)` or
`tag.main` which IMHO makes the code less consistent.
The Capybara::DSL monkey-patch is only done on the `visit` method
because it's the only reliable one. Other methods like `click_link`
generate AJAX requests, so `expect(page).to have_css "main", count: 1`
might be executed before the AJAX request is finished, meaning it
wouldn't properly test anything.
