Improve board history readability

[twschiller]

Board history was originally implemented as part of #61. However, the current history is confusing. It could be improved as follows:

• Report the initial values as their initial values, not as edits
• When evidence and hypotheses, state that they were added, not edited
• Group edits that were performed at the same time
• When evidence and hypotheses are removed, state which one was removed

Additionally it might be helpful to view the history of a particular hypothesis or piece of evidence.

[nnrepos]

in this sentence: When evidence and hypotheses, state that they were added, not edited did you mean: When evidence and hypotheses are added, state that they were added, not edited?