Fix images wider than 100%, and use lightbox

[ctrueden]

It used to be that images were not allowed to be larger than 100% in width. They'd scale down as the page got narrower. Unfortunately, this had negative side effects inside tables, since 100% means something different than "width of the main content pane" when inside a table cell.

We need to find a way to make images max 100% of the main content pane by default again. Here is an example page that looks bad now because of it:

https://imagej.net/scripting/how-to-use-the-javadoc

It would also be ideal to make such images linked to their full-resolution versions, which pop up using lightbox when clicked. The thumbnail include (slated to be reworked) currently does this I think; see #154 for related work to be done.