Make and manage persistent observable comments

[seanmcfeely]

Sometimes you figure something unique out when working an alert, and it would be valuable for a comment to persist while it's relevant so that other analysts don't have to re-do analysis work.
For example, if an analyst discovers that a certain user uses a VPN service on his/her system then the following comment could be added to the relevant user observable:

This user uses [example VPN solution ](https://fake.vpn.example.com) on his/her system.
As discovered through these details:
        - detail
        - detail

Also, blah blah blah.

^ Make the comments so they can be expressed in markdown. This will add flexibility on the presentation. These comments should be editable directly in any alert where the observable is found.