Improve automated changelog generation

Updates to the changelog are generated based on commits. There are a few things that can be improved about the generation process to make this even easier. Below is a list of things that were done manually on the first real test (see !735 (merged)):

• Removed extra empty lines between each pair of commits listed.
• Added cta/ before every commit and removed extra whitespace.
• Replaced incomplete commit messages (with ... at the end, instead of the full message) by it's correct text.
• Removed the date from the top header of the new version.
• Reordered all entries by the number of the issue.
• Manually moved the entries that were not correctly categorized.

Not all of these can be automated (in particular the last 2 will be tedious/impossible), but we should have a look to improve things where we can.