Improve meta-programming and display around informal results

[jstoobysmith]

There are two parts to this issue:

• The way informal_lemma and informal_definition currently work in HepLean is that they are turned through elaboration into elements of the type InformalLemma or InformalDefinition. This does not seem like the most optimal approach, and an approach similar to TODO elements will likely work better.
• The output of informal_lemma and informal_definition on the HepLean website needs improving. Either with an updated form of graph or just a list of them.