consolidate hub-components dependencies

[tomwayson]

We should consider consolidating all packages that hub-components dependencies on into @esri/hub-common.

This will allow us to streamline the process adding or updating components by, b/c when we release Hub.js we'll only need to bump @esri/hub-common in the Hub app, and we won't need to be cognizant of any manual peer dependency bumps.

Consolidate the following into common:

• events - this is a small and stable package, last updated over a year ago, only used in one place in the Hub app
• search - outside of hub-components, this is only used in one place in the Hub app
• downloads? - only used by hub-components, not the Hub app, so not currently as hard to manage as the other, but its also small
• discussions - this is a very small package that is not yet used by hub-components, but w/ the active development in this area I'm sure it will soon.

Teams will be taken care of by #655