Package Manager: Add button for installing dependencies

[lachlanjc]

(Especially useful after the upgrade all button.)

[eveningkid]

I think this is not a good idea for two reasons:

• the package manager isn't meant to be something messy. It shouldn't have too many buttons (it already has two)
• every time you add/remove a package, there is no need to call yarn install again, it deals with it automatically