Allow admins/masters to delete (some) system notes

Description

We are human and sometimes we make mistakes. Sometimes we paste in the wrong link or type the wrong issue number. Sometimes we enter the wrong date for time tracking, or we made a typo in the duration.

Sadly currently either you get misleading cruft in the notes that end up confusing everyone, or worse, you get wonky negative times for the sake of fixing incorrect data.

To circumvent this, currently on our instance we:

1. click on a mistaken note to get the anchor which contains the id
2. ssh on our instance, fire up gitlab-rails console
3. Note.find(id), review and .delete
4. redo what was wrongly done

Proposal

At the very least, 2+3 should be possible straight from the UI for sufficiently privileged users (I'm thinking master and admin).

Obviously for time tracking notes, the tallies should be updated upon deletion.

Links / references