Enable users to "watch" a dandiset

Long term item.
Inspired by the utility and success of our "fix the OHBM conference poster session" (visit e.g. https://datalad-datasets.github.io/ohbm2020-posters/index_beta.html#/) -- I thought that it would be nice to eventually add functionality for any user to start "watch"ing a dandiset (like github does for repos) and receive announcements on new releases (or even optionally updated to drafts)

(See also dandi/dandi-api#703 for a webinar feature that would make use of this watch functionality.)