Add link to the HECO app

[sirbeefalot]

Context

While we develop the next version of the Beefy app, or add native a multi-chain feature, the other chain where we have farms is in a different subdomain at https://heco.beefy.finance

## Problem
Users might not check the farms on that chain, or even know that we have farms on HECO already.

Solution

It would be good to have a link from our main app into the HECO app. The challenge is how to do it without making the header even more cluttered than what already is.

[sirbeefalot]

Closed with #220