Improve web performance

* Defer the downloading of the `YouTube player` and `tweet buttons`
  related code until after the page completely loaded.
* Remove the `logo.svg` reference (see: #105) from `index.html`, as
  it adds an extra HTTP request, without bringing much benefit.
* Remove the `logo.svg` (see: #105), as it only adds an extra HTTP
  request, without bringing much benefit.
* Specify image dimensions.