[Package Renames] Add feature flag to disable popularity transfers

[loic-sharma]

Adds the feature flag Search.TransferPopularity that, if disabled, removes all popularity transfers on the search index. Note that download overrides are still applied even if the popularity transfer feature flag is disabled.

Addresses NuGet/NuGetGallery#7944
The feature flag staleness monitoring is tracked by NuGet/NuGetGallery#7966

[shishirx34]

nit: personally I feel an enabled check is more readable and intuitive than not enabled..

[loic-sharma]

I prefer the negative pattern as it avoids nesting if there are multiple conditions that must be met before returning the result. For example:

if (condition1)
{
    if (condition2)
    {
        return data;
    }
    else
    {
        return errorResult2;
    }
}
else
{
    return errorResult1;
}

return emptyResults;

Vs:

if (!condition1)
{
    return errorResult1;
}

if (!condition2)
{
    return errorResult2;
}

return results;

Please let me know if you feel strongly about this.