issue #659 make build platform independent by using cross-env

Upgrade dependencies to fix compatibility with Node v10

Upgrade dependencies using `yarn upgrade`. This fixes dependency
installation with Node v10.

change links in help panel

"send a suppport ticket" now leads to `https://web.hypothes.is/get-help`
and "visit our help documents" leads to `https://web.hypothes.is/help`

Fix group focus change checks

The focused group change check had several issues:

 - The `prevFocused` variable was not updated after the focused group
   changed, so the `GROUP_FOCUSED` event was emitted every time the app
   state changed.

 - Groups were compared by reference rather than by ID, which means that
   a change which modified a property of the group but not its ID would
   cause the event to fire.

The first issue made it impossible to create annotations because the
unnecessary `GROUP_FOCUSED` event triggered a cascade of reactions which
crashed the app.

Modernize PDFMetadata class

 - Convert `PDFMetadata` to an ES2015 class

 - Convert the ad-hoc type documentation to tool-readable JSDoc

 - Add a little more detail in doc comments about what
   `documentFingerprint` is

Move focused group and list of loaded groups to store

As noted in CR feedback, having a selector named `focusedGroup` which
returns a `Group` and a state property named `focusedGroup` is
`string|null` is confusing.

Also change a `g` variable to `group` for clarity.

Also bring `yarn.lock` into sync

Remove outdated April fool joke.

Use group.links\.html instead of group\.url

Remove used used scss block causing invalid css error after upgrading…

Fixes: https://github.com/hypothesis/product-backlog/issues/542
Also see: https://github.com/hypothesis/product-backlog/issues/486

Add organization logos to groups menu

Resolve relative URLs when getting PDF URL from PDF.js

If PDF.js is launched with a `file` query-string parameter that is a
relative URL, then `window.PDFViewerApplication.url` will be that
relative URL.

When capturing the metadata from the PDF in order to search for
annotations or find relevant groups, we want to use an absolute URL.
This commit uses the existing `normalizeURI` helper to resolve the
possibly-relative URL.

Fixes https://github.com/hypothesis/product-backlog/issues/579

Extract the existing `normalizeURI` function from guest.coffee into a
utility module and re-use it in document.coffee to convert relative URLs
to absolute ones.

Also implement some direct tests for this function.

Update the groups service to use the Redux store for storing and reading
groups state instead of storing it internally in private variables.

Add a Redux module that stores state related to groups, including the
currently focused group and the list of all known groups.

This is an initial step towards moving this state from the groups
service into the store, which will make reacting to changes in the set
of loaded groups, or the focused group, easier.

Extract `createStore` helper out of `store` service

Replace "inherits" package with ES 2015 class inheritance

Convert Adder to an ES2015 class