fix(perf): change css link tag's initial rel attr to preload

[chenba]

Because:

• we don't want the CSS request to block rendering

This commit:

• change the link tag's initial rel to preload and switch to stylesheet
  on page load

[chenba]

Note that this PR includes the commit from #19004. The original PR was also reverted with #18955 because I believe that it broke stage. I made the changes in #19004 to verify the patch in this PR and it appeared to work.

[chenba]

@vbudhram since you reviewed #18933, wanna rubber stamp this one? The code hasn't changed. But I re-verify it after #19004.