Remove the hypothes.is/stream page

The legacy "stream" page at should be removed. This page hasn't been used since we introduced the new page a long time ago (I don't think there are any links to `/stream` anywhere anymore).

The `/stream` page, even at its peak when we were actually using it as on of the primary pages of Hypothesis, always had a lot of bugs.

Because of the way it's implemented (the client sidebar code is reused to provide this standalone page) the stream page often receives a lot of bugs - changes made to the sidebar accidentally break the stream.

Because the stream is now legacy and no one uses it, these bugs go unnoticed and pile up.

Some issues that can currently be observed on :

1. _Message not available_ cards instead of realtime annotations


2. The username of the logged in user, instead of the annotation's creator, appearing on annotation cards

3. Reply buttons and functionality extremely badly broken


4. Scrolling completely broken


5. Third-party annotations, which don't work properly on the stream, are nonetheless appearing on the stream.

6. Probably a lot more problems.

Some of these issues are [over a year old](https://github.com/hypothesis/client/issues/123).

See also: [Slack discussion](https://hypothes-is.slack.com/files/U074DEU66/F897NB2BU/screenshot_from_2017-12-04_13-22-50.png).

It would take a long time to fix all of the above bugs, and for the reasons cited above more bugs are just going to appear anyway, so my suggestions is that we just remove the stream page.

Start by locating the route and implementation for https://hypothes.is/stream, then search for remaining links or references to /stream and compare with the newer /search page. Done means the legacy page is removed without affecting /search or the client sidebar; run the relevant project checks after the change.

Written by the indexing model from the issue text.