Align visually hidden behaviour with GOV.UK Frontend

[colinrotherham]

There are a number of issues against visually-hidden behaviour:

• Summary list visually hidden text on Android #704
• NVDA not reading visually hidden text on mouseover #760
• visually-hidden sets a property that visually-shown doesn't reset #1168

We should see if these are resolved by GOV.UK Frontend

Visually hidden

For example, we can see various visually-hidden changes in:

• Add important parameter to visually hidden sass mixins alphagov/govuk-frontend#1037
• Update visually hidden class to fix ordering issue in VoiceOver OSX alphagov/govuk-frontend#1109
• Announce whitespace in screen reader announcements of visually hidden text alphagov/govuk-frontend#3836
• Refactor govuk-visually-hidden Sass alphagov/govuk-frontend#5063

Visually shown

But we should likely deprecate and remove visually-shown (see #1168) to prevent conflicting styles.

I'd recommend a targetted visually-hidden instead:

Before

@include visually-hidden; // [1]

@include mq($from: desktop) {
  @include visually-shown($display: table-header-group); // [2]
}

After

@include mq($until: desktop) {
  @include visually-hidden; // [1]
}