Layout: Change Sidebar to Drawer

[timmyc]

The final direction for the <Sidebar /> area of the app is for it to now act as a drawer - akin to the masterbar notifications panel on WPCOM. The desired functionality can be seen and interacted with here - but a .gif is worth a thousand words:

Naming
As part of this task, I was wondering if we should consider renaming the containing component from Sidebar to something more in-line with what we refer to this feature as. The few names that come to mind for me are:

• Activity Panel ( seems singular )
• Activity Drawer

But I'm totally open to any and all ideas. Sidebar sounds a bit too much like the wp-admin sidebar nav that is always shown, vs this drawer that is accessible on all Woo screens.

Changes
This task definitely has some tie-in to #90 so perhaps this could be done along-side that or after. But the big changes needed are:

• The sidebar/drawer no longer shrinks the content in div#woocommerce-layout__primary it overlays it
• The sidebar/drawer by default is not exposed
• Again in conjunction with Timeline: Update Tabs #90, the tabs control the display of the drawer. The only way the drawer can be closed is to click on the open tab again

/cc @jameskoster for any further interaction details that I may have missed

[jameskoster]

FWIW we've been calling it the Activity Panel in our discussions :)