Highlight addition/deletion in "Title changed" system note on issues/MRs

When the issue or MR title is changed, a system note is posted showing "Title changed from old title to new title". It would be nice to highlight the part that changed, like we do with file renames and inside diffs: https://gitlab.com/gitlab-org/gitlab-ce/commit/91cdb08d5fa83a86f5f18088bf148e832732bed9

We could support the [-deletion-] and {+addition+} syntax in GitLab Flavored Markdown, and use this syntax in the system note.

cc @rspeicher