Web IDE markdown preview should have a max width

The Web IDE markdown preview should be have a max width and be center aligned, so that on a large screen the preview remains readable. The max width 990px, the same as the markdown rendered in the repository readme.

Proposal

Make the md preview centered and max-width 990px

.preview-container .md-previewer {
  max-width: 990px;
  margin-left: auto;
  margin-right: auto;
}