Remove share widget exclusion for elife

## Feature Request Form
### Problem you are trying to address with this feature

There is no share button for the elife implementation.

This commit:
https://github.com/hypothesis/client/commit/d11be3fb0ac168d80c65603bad4b735bd455200b

Removed this capability, but only for their authority. Per the commit message:

> Don't show the "Share this page" button in the top-right of the sidebar
> on pages, such as eLife's, that have both an embedded Hypothesis client
> and a services config including their own `authority` string.
>
> The "Share this page" button is a Via link, and Via doesn't work with
> pages using a third-party authority config.

### Your solution

Now that we use bouncer links in the share widget, as of this PR https://github.com/hypothesis/client/pull/748 and also now that we can whitelist sites embedding H per this PR https://github.com/hypothesis/bouncer/pull/155, we should remove this exception.

Also, note that we will presumably need to add elife to the bouncer/embed_detector.py at the same time.

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