Make private highlights less confusing

### Problem you are trying to address

Users clearly find the idea of highlights being private confusing.

#### Zendesk issues

- https://hypothesis.zendesk.com/agent/tickets/27
- https://hypothesis.zendesk.com/agent/tickets/28
- https://hypothesis.zendesk.com/agent/tickets/235
- https://hypothesis.zendesk.com/agent/tickets/482
- https://hypothesis.zendesk.com/agent/tickets/803

#### Other user reports

@chrisaldrich in https://github.com/hypothesis/h/issues/4229:

> Currently it appears that highlights default to "only me" after they're highlighted. Annotations appear to default to public initially as they're being written, but when re-entering the text without manually hitting publish, they then are posted automatically to "only me".
>
>This may be a reasonable default, but it means spending more time with the interface while I'm reading/working and thereby taking me out of the flow of my work.
>
> Unfortunately, there is no default setting for forcing my highlights and annotations to be public, which means when I'm done reading that I have to go into the UI and manually do three button presses to make this happen for EVERY post. Click to edit, click the setting arrow, and finally click to publish. UGH!

#### Metrics

As of 2017-02-22 [we have nearly 6000 annotations with only a single character in their text field](https://metabase.hypothes.is/card/38).

As of the same date, we have 2,127 annotations which have only whitespace content, and 874 which contain only a single full-stop character.

This isn't counting annotations where tags instead of content have been added to allow them to be made public.

----

Whether or not we want to allow public highlights, it seems that we're not doing a great job of explaining to users why highlights are private by default, and we're not doing a great job of actually preventing users from creating content-free annotations regardless -- many users have worked out that just putting a space or a full-stop in the annotation body will allow them to publish an annotation publicly.

Start by reviewing the linked Zendesk reports, the referenced issue, and the reported annotation metrics. Clarify whether the goal is to change highlight visibility defaults, better explain private highlights, prevent content-free public annotations, or some combination. Done should include an agreed user experience and measurable handling of the reported cases.

Written by the indexing model from the issue text.