Add lines_of_code feature flag

What does this MR do and why?

Related issues

#795 (closed) #895 (comment 503444)

Add lines_of_code feature flag to determine whether to display lines of code on the project contributors page.

Frontend also can check the state of the feature flag

if ( gon.features.linesOfCode ) {
  // ...
}

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.