Updating client dependencies. Switching to yarn. (#6433)

* Updating client dependancies. Switching to using yarn.

* Updating React

* Moving pure components to using function syntax (performance gains with newer react version)

* Updating client dependancies.

* Ignore .yarninstall

* Enabling pre-lockfile because it's the entire point of using yarn.

* Removing old webpack config

* Moving to new prop-types

* Fixing ESLint Errors

* Updating jest snapshots.

* Cleaning up package.json

.gitignore

@@ -22,6 +22,7 @@ cmd/mattermost/imports.go
 # Build Targets
 .prebuild
 .npminstall
+.yarninstall
 
 # Compiled Object files, Static and Dynamic libs (Shared Objects)
 *.o