Change tab size from 8 to 4 spaces in the Web UI (BB-7457)

When developers use tab for indentation, they mainly do that to have the indent size in their IDEs adjustable (to configure it as one prefers). Usually people use 2 or 4 spaces for tabs in the editor. Currently BitBucket converts tabs to 8 spaces to show them in its Web UI. It does not look convenient, especially when you have a couple of nested blocks in your code. It would be much better to have 4 spaces for tabs on the Web, not 8 (too much for indentation).