Store issue / MR description changes
Every time the description of an issue / MR changes, we need to keep track of the change by storing the diff or storing both old and new versions.
We also need to associate this with the system note regarding the updated description so that we can display it there in the future.
This is the first step in implementing https://gitlab.com/gitlab-org/gitlab-ee/issues/10103
Some things to note from previous discussions:
- If saving in DB, we should save the data in a separate table (not in notes) and we should probably only save the diffs https://gitlab.com/gitlab-org/gitlab-ee/issues/10103#note_146020967
- We should handle combined description edits for consecutive edits within a short period https://gitlab.com/gitlab-org/gitlab-ee/issues/10103#note_146020980
- Although this is a GitLab Starter feature, we want to save changes for everyone so that they will be available when upgrading https://gitlab.com/gitlab-org/gitlab-ee/issues/10103#note_201216686