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.
Since the message might appear several times on the same page, it's
useful to give a bit more context. Besides, usability tests show that
when there's a group with no headings, there's no clear indication on
the page that the group is actually a group and not a heading.
We're also adding some emphasis to the group name in the "Showing
headings" message, to be consistent with the emphasis we've
added the the group name in the "No headings" message.
Captions benefit blind screen reader users who navigate through tables,
particularly in this case because we potentially have several tables
with headings (one table per group), so when navigating through tables
it might be hard to know which group the headings belong to.
In this case they also benefit sighted users. Usability tests have shown
the "Groups and headings" section is a bit confusing, so adding a
caption like "Headings in Districts" helps clarifying Districts is a
group and the table refers to headings in that group.
The very same table is rendered in the "headings" step of the budget
creation wizard. However, in that case the information of the caption is
redundant because the page is specific for headings belonging to a
certain group. We're making the element invisible but still keeping it
for screen reader users in order to ease their navigation through
tables.
