Track WikiPage edit history

[chaoran-chen]

Based on #88 and following the wiki philosophy, the change history of a WikiPage should be stored. For this issue, we can choose between different options:

• Simplest: Only store the name of the very last person who edited a page. (This was implemented in Edit wiki page #91. The issue will be kept open in case that we would like to implement one of the more sophisticated options.)
• Better: Store the author names and timestamps of all edits.
• Perfect: Store the changes as well.

Independent of which option is chosen, there are the following tasks:

• Extend the "edit wiki page" controller (or service) and store the additional information into the database.
• Extend the "add wiki page" controller (or service) as well (since the initial author is the first editor)
• Add a GET endpoint that returns the change history of a WikiPage. To reduce data privacy issues, I suggest that the endpoint should only be available for logged-in users.