Replace "«" with left chevron PNG icon in "Back to X" navigation links

[DrGFreeman]

Status

Ready for review.

Description of Changes

Fixes #5596

Changes proposed in this pull request:

Replace the "«" character in different "Back to X" navigation links with a left chevron PNG icon. The icon is the same as the one already used in the JI (in source breadcrumb) except in the opposite direction.

Alt tag of icons is set to "" so they are ignored by assistive technologies, as per WCAG recommendations for decorative images.

Testing

How should the reviewer test this PR?
Write out any special testing steps here.

1. In the Source Interface, navigate to /why-journalist-key.
   • Verify the left chevron icon is present left of the "Back to submission page" link.
2. In the Journalist Interface, access the Admin interface and click on "Add User".
   • Verify the left chevron icon is present left of the "Back to admin interface" link.
3. From the admin interface, click on the "Edit" button of one of the users.
   • Verify the left chevron icon is present left of the "Back to admin interface" link.
4. From the admin interface, click on the "Instance Config" button.
   • Verify the left chevron icon is present left of the "Back to admin interface" link.

Screenshots of expected results are included below for reference.

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing production instances.
2. New installs.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[DrGFreeman]

Reference screenshots

Source Interface (/why-journalist-key)

Admin Interface - Add User

Admin Interface - Edit User

Admin Interface - Instance Config

[rmol]

👍 Checked in Tor Browser, looks good. New icon is included in the Debian package. I'm approving, and will kick CI to see if we can get past the flake, but if not, it should just take a rebase onto latest develop to be ready to merge.