Release posts: add buttons for doc, issue, and MR

What does this MR do?

• Closes #2437 (closed)
• Makes the already existing links to issues, MRs, and epics available for the readers of release posts through buttons at the end of each feature block
• Make a button for webpage link (separate from the doc link)
• Deprecates the use of documentation_text
• For #2437 (comment 90265520)

Preview

Review app: https://release-posts-add-issue-mr-buttons.about.gitlab.com/2018/07/22/gitlab-11-1-released/

Buttons available	Feature block	Performance improvements

Checklist

• Add doc reference to a button (we won't need documentation_text anymore)
• Add button for `issue_url``
• Add button for mr_ulr
• Add button for epic_url
• Add button for webpage_url
• Approval from Mike and Job
• Jarek's review

To fix:

• Border color (a little lighter)
• Performance improvements links
• Update handbook and templates
• Revert tests for previewing/reviewing
• Update 11.2 MR