Allow a user to set a global default for whether they want to post publicly or privately

### Problem you are trying to address

Allow a user to set a global default for whether they want to post publicly or privately.

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

> Perhaps it exists already and I'm not seeing it, but most of what I highlight and annotate, I typically prefer if it were automatically published freely and in the open. 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!
>
> In addition to allowing a master setting toggle for public/private at the main account level which would alleviate a large portion of my issue, I'd also recommend that rather than a drop down menu with two options that there just be a simple toggle to switch between public/private on each post so the user can accomplish in one click (or fingerpress on touchscreens) what currently takes two. Admittedly it's also controlling a "publish" function simultaneously, but I think that each post could have a public/private toggle and a separate "publish" button without cluttering up the UI too much.

(emphasis editorial)