Client does not respond to `data-hypothesis-trigger` elements added to page after client loads

Issue originally reported here: https://hypothesis-open.slack.com/archives/C6Y375TT3/p1606558165041600

> I have a button with "data-hypothesis-trigger" but this does not seem to work when the page is changed using javascript. Anyway to manually trigger the sidebar open using javascript?

**Steps to reproduce:**

1. Add an element to the page with the `data-hypothesis-trigger` attribute set after the client has loaded.
2. Click on the element

**Expected result:** Sidebar opens
**Actual result:** Sidebar does not open

**Notes:**

If the element is on the page before the client loads, the button should work.

The code that [installs the trigger](https://github.com/hypothesis/client/blob/09a586c2d4699b9ff92104171f833ba550aea399/src/annotator/sidebar-trigger.js#L11) does so directly on the elements in the document with this attribute set and only once during client startup.

We could avoid this problem using event delegation.