Fix client failing to load on pages that define `self` to be something
other than `window`.

When AnnotatorJS is bundled by browserify, it is first processed by
browserify-shim* which adds references to a variable called 'global',
which exists in Node but not the browser. Browserify then processes the
result and assigns `global` to the value of the first variable from
['global', 'self', 'window'] which is already defined.

https://github.com/hypothesis/h/pull/3385 fixed the problem when the
page defines a variable called 'global' that is not an alias for
'window' by reducing this list to ['self', 'window']. This still leaves
pages that define 'self'.

This commit fixes the problem by always making `global` (within CommonJS
modules) always be assigned the value of `window`.

* browserify-shim deals with the fact that AnnotatorJS 1.2.x is not a
CommonJS module and exports its entry point as `window.Annotator`.