Improvements to "Only Me" annotations

_This is a record of a feature request sent to our support address: [#836](https://hypothesis.zendesk.com/agent/tickets/836)._

> On a page, after you load the bookmarklet, you will see all the annotations from that page, filtered based on the selected group (and you can choose between 'Public' or any other group you are part of). Any plan to add a new option there? "Only mine" - to see only mine annotations from that page?

This is consistent with regular feedback that we get where people are frustrated that Only Me is inside of individual groups instead of a universal layer. (Prior context: https://github.com/hypothesis/product-backlog/issues/200 and also https://github.com/hypothesis/product-backlog/issues/528) This leads to usability issues, where they are confused about where their annotation is, and whether it's public or not, such as this issue: https://github.com/hypothesis/h/issues/5111

Should we allow you to set your view into a personal (Only Me) mode such that any *new* annotations you made would also be Only Me by default, and so that you could see all your Only Me annotations together without any others distracting you? Should we also allow you to view your "Only Me" annotations from our search interface (activity pages) wherever they were made? Since the group control is sticky (stays in the group you set it to, even across sites), you would be able to maintain that setting across sites, which has also been a request.

This is also related to the concept of an "All" scope or "group", where a user could see all annotations from any group on a page without having to click into each one. (https://github.com/hypothesis/vision/issues/207)

No source files, tests, or entry points are named. Start by reviewing the bookmarklet's group selector and the search interface/activity pages, along with the linked prior issues, to clarify whether the scope is a personal Only Me view, default annotation visibility, cross-site persistence, search support, or an All scope; done requires an agreed, testable product scope.

Written by the indexing model from the issue text.