Make code-like content findable in epics for advanced search

What does this MR do and why?

This MR switches the epics.description field's analyzer from standard to code_analyzer. We believe that it's going to improve search quality. Further I have renamed and moved the shared context with elastic code examples from ee/spec/lib/gitlab/elastic/search_results_spec.rb to the shared_contexts/ for reusability.

MR acceptance checklist

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Before	After

How to set up and validate locally

• Make sure Elastic search is enabled

On the master branch:

• Create an Epic with any title and description dots.also.need.testing
• Make sure this new epic is available on Elasticsearch. You might need to wait a bit or sometimes restart the rails-background-jobs
• Now perform a group search under the epic's group with the keyword also.need
• You won't find the created epic in the search results

Now switch to the MR branch

• Run the bundle exec rake gitlab:elastic:index to reindex the epics with the new analyzer
• Make sure this new epic is available on Elasticsearch. You might need to wait a bit or sometimes restart the rails-background-jobs
• Now perform a group search under the epic's group with the keyword also.need
• You will find the created epic in the search results

Related to #435050 (closed)