Add long and medium term goals for the web team

Created by: felixfbecker

Adds our team's long and medium term goals to the handbook. For context on how we arrived on these goals, see this document.

These goals will guide our short-term iteration goals and tell the story of why we are doing what we're doing.

In addition, I reworded the ownership section to be derived from our long-term goals instead of from what other teams don't own (our ownership area doesn't change from this, it's just less ambiguous). This has been a point of confusion before.