Ordered list in assignment editor

[torgeirl]

The assignment editor currently only support unordered (bulleted) lists (<ul>), but as described in #78 assignments are often split into sub-assignments. When an assignment text is organize using either numbers or letters the text get structured like a list but it will lack proper list semantics.

In addition to adding support for sub-assignments (not planned), we should also add support for ordered lists (<ol>) with either numbers (type="1") or letters (type="a" or type="A").