Resolve "Disable the experimental GitLab elasticsearch indexer checkbox if it's not in $PATH"

What does this MR do?

Disables the "Experimental elasticsearch indexer" checkbox in the UI if the gitlab-elasticsearch-indexer binary isn't in $PATH, and falls back to the traditional Ruby indexer if it isn't present.

This will improve the user experience for people who install from source, or in case omnibus-gitlab#2168 (closed) is missed.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Uninstalled:

Installed:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2149 (closed)