SPIKE: Add annotation counts to scope control

**NOTE: the groups dropdown has changed quite a bit since this issue was open, so we've changed this to a Spike. See [this comment](https://github.com/hypothesis/product-backlog/issues/260#issuecomment-523545066) for details.**

### User Story

As a user of a publishers site, I want to know the number of annotations available in a particular channel, to see how many annotations on a page in all the groups that are available to me.

### Background
Currently, when you activate the sidebar on a page, the only indication of the # of annotations that are available on the page come from two sources:

1. Badge - the extension icon has a number badge that appears anytime there are annotations available on a page.

2. Once you drill down into a group/channel and select that channel, you will see the # of annotations/page notes/orphans made on that page.

### Problem with current approach
If a user is a member of many groups, or, if the user is a member of a 3rd party group, and a Hypothesis group, they don't have an easy way of knowing what annotations are available to them in each group, in an easy to comprehend way, without clicking through each of the channels individually to figure it out.

### Suggested Design

### Acceptance Criteria
- Add annotation counts next to each individual channel
- All users, both logged in, and logged out users should see the counts (and annotations) of annotations publicly readable to them `world-readable` (i.e. the public channel, and publisher's world-readable channel)
- Logged in (authenticated) users should see counts of channels available only to them (private groups, private annotations).

### Outstanding issues
- [ ] Are replies included in the counts? (Answered below: No - only annotations + page notes are included in counts)
- [ ] Is there anything visible in the sidebar UI or host page that depends on the counts apart from the content of the dropdown menu? (Yes - there is a button/link on the host page that shows the # of annotations made on the page)
- [ ] Do we use this endpoint for the annotation counts in the host page? (See below)