Guidance around opening links in new tabs

[marteki]

Updates the Design Manual to detail the situations when it's more desirable to open links in a new tab.

Additions

• Detail out behaviors for when to open links in new tabs
• Include wording around code to prevent security problems
• Include guidance and direction for screen reader accessibility

Review

• @sonnakim
• @ielerol
• @Scotchester
• @cfarm
• @StephanieLevy

Preview this PR without the whitespace changes

Checklist

• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows the standards laid out in the front end playbook
• [x] Passes all existing automated tests
• [x] New functions include new tests
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged
• [x] Visually tested in supported browsers and devices
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[sonnakim]

@marteki Agreed with @Scotchester 's suggestion to add a code example for screenreader-only text. Otherwise 👍

[marteki]

@Scotchester @sonnakim Does the added code example work?