[OYPD-477] Gallery next/previous buttons are not keyboard operable.

[ivan-berezhnov]

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General checks

• All coding styles are fulfilled and there are no any issues reported by CodeSniffer CI.
  
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Thank you for your contribution!

Jira issue:

https://propeople-us.atlassian.net/browse/OYPD-477
Drupal.org issue: https://www.drupal.org/node/2885460

Steps to review:

• Go to [sitename]/locations/west-ymca
• Click "Tab" on keybord
• When you have focused on the Left Arrow or Right in the Gallery
• Verify you can see blue border near the Left Arrow or Right buttons

[gundevel]

Build comment file:

===============

Environment	Link
Fresh OpenY installation	http://ci.openymca.org/build1531
Upgraded(upgrade path) installation	http://upgrade.openy-dev.ffwua.com/build1531
Installation process	http://install.openy-dev.ffwua.com/build1531/install.php

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Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://ci.openymca.org:8080/job/PR_BUILDER/1531/

[gundevel]

Behat test results file URL: http://ci.openymca.org/build1531/profiles/contrib/openy/build/reports/behat