[WIP] Add/expand collapse sidebar

[cpapazoglou]

Changes proposed in this Pull Request

• add CollapseSidebar sidebar item component
• add sidebar collapsed status to state and create a getSidebarIsCollapsed selector
• add tests for reducer and actions
• tries to add a class in the layout component depending on which sidebar will collapse or not

Testing instructions

[matticbot]

Test live: https://calypso.live/?branch=add/expand-collapse-sidebar

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~115 bytes added 📈 [gzipped])

name        parsed_size           gzip_size
entry-main       +414 B  (+0.0%)     +115 B  (+0.0%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Async-loaded Components (~134 bytes added 📈 [gzipped])

name                                          parsed_size           gzip_size
async-load-my-sites-sidebar-unified-switcher       +434 B  (+0.2%)     +134 B  (+0.2%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[cpapazoglou]

Using state to store the sidebar status (expanded / collapsed) in order to use for rendering the layout here
https://github.com/Automattic/wp-calypso/pull/45989/files#diff-e8ab61833a6d85e586cf195046ed6f15
leads to re-rendering the biggest part of the app thus creating performance issues. I will leave this as a draft and explore another way to accomplish it.