Allow users to write and save comments on a particular time window

Often times I use profiles to work out an understanding of how a large process works, and I want to go through it step by step and collect notes on what's going on. Currently the only way to do that is to have, say, a google doc with a bunch of links to different filters on a profile. It would be really nice if I could highlight something in the profiler, right click, and add a comment. It seems like this could be nice for sharing profiles, too - it could help highlight the piece that you're asking for help with, or explain away something so that others don't go down the rabbit hole of investigating it.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-244)