Move Redux modules and entry point to `store/` directory

Changed background to selected element in PDF.js to visualize the searched content

Remove reference to `group.url`

Remove `group.public` property from annotation component

Update icon for restricted groups in publish-annotation-button

Move away from `public` property in application components

Keep the structure of having modules at higher layers (ie. with more
transitive dependencies) in the app nearer the top of the list by moving
the store registration to the bottom.

Following the plan outlined in #687, restructure the Redux store as
follows:

 - Move the entry point from `services/annotation-ui.js` to `store/index.js`
 - Move the modules which define bundles of reducers, action creators
   and selectors to `store/modules/{module-name}.js`
 - Move related utility code to `store/{module-name}.js`

Move sidebar services to src/sidebar/services/

Following the plan outlined in #687, move services into a `services/`
directory.

The definition of "services" in that plan (a possibly stateful module
that handles interaction with an external party) is slightly more narrow
than what constitutes an Angular service (a stateful singleton that has
dependencies on injectable services/values).

However as a first approximation most Angular services are services as
defined in #687, so I've simply moved them all to the `services/`
directory.

Rename src/sidebar/{store => services/api}

Give the service which provides a client for the Hypothesis API a more
obvious name and place it in a `services/` dir in accordance with the
plan outlined in #687. The previous name of "store" was particularly
confusing because of the existence of another kind of unrelated store
(the Redux one) in the application.

The service was previously registered with the `factory` method but has
the same structure as all the other services which are registered with
`service`, so I changed that for consistency.

Show the world icon only for open groups in the groups dropdown.

ADR: Sidebar application architecture

private group icon otherwise.

Fixes https://github.com/hypothesis/product-backlog/issues/513

Default to notes tab when only notes present

`selectedTab` is set to "note" rather than "page" if only page notes are
present.

This adds two tests that verify the annotations tab is selected
when there are annotations present and the page notes tab is
selected when only page notes are present.

Change the default tab to focus on page notes when there are
no annotations and only page notes in a group. This way it
directs the user where they need to go. Less clicking == yay!
If there are no annotations at all, the annotations tab should
be selected.

Allow multiple state modules to handle the same action

In some cases multiple state modules may need to handle the same action.
For example, when annotations are loaded or unloaded we may want to
update both the set of loaded annotations and also the current
selection.

This commit changes the `createReducer` helper and the code that uses it
in `reducers/index.js` so that multiple state modules can define update
functions that handle the same action.

Note that all update functions get the same, _previous_ state as input
and there is no guarantee on precedence if they return conflicting
updates.

See also: https://redux.js.org/faq/actions#is-there-always-a-one-to-one-mapping-between-reducers-and-actions

Remove activity page links for third-party open groups

Activity pages are currently only supported for open groups belonging to
the h service's first party authority.

The team has agreed that the best way to resolve this would be to only
include the "activity page" link in the `/api/groups` response for
groups where that is supported. However to give us more time to hammer
out the API details, we are committing a small change to the client in
the interim.