Remove or document SafeMarkup::set in search_excerpt

Problem/Motivation

search_excerpt calls SafeMarkup::set() which is meant to be for internal use only.

Proposed resolution

• Remove the call by refactoring the code.
• If refactoring is not possible, thoroughly document where the string is coming from and why it is safe, and why SafeMarkup::set() is required.

Remaining tasks

1. Evaluate whether the string can be refactored to one of the formats outlined in this change record: https://www.drupal.org/node/2311123
2. Identify whether there is existing automated test coverage for the sanitization of the string. If there is, list the test in the issue summary. If there isn't, add an automated test for it.
3. If the string cannot be refactored, the SafeMarkup::set() usage needs to be thoroughly audited and documented.

Manual testing steps (for XSS and double escaping)

Do these steps both with HEAD and with the patch applied:

1. Clean install of Drupal 8.
2. create a node, enable search module, run cron, and search for a word in your article
3. Compare the output of the snippet above in HEAD and with the patch applied. (You should see strongs wrapped around the keywords you searched for). Confirm that there is no double-escaping.
4. If there is any user or calling code input in the string, submit
   alert('XSS');

   and ensure that it is sanitized.

User interface changes

N/A

API changes

N/A