Manually remove bootstrap's tooltip before re-rendering.

- Since we put tooltip directly under `body',
  there's some rare case when tooltip is activated just
  when `Shreds.render' called. Resulting in the tooltip
  lingering undeleted. This tooltip may stacked-up like
  garbages, so we need to remove it by any means.

- Also refactor feeds rendering to use `document.documentElement.scrollTop'
  if possible.