How can I mount my phone and my battery pack to my bike?
How about a top tube bag? 

I've had a couple of handlebar mounts and they haven't lasted long in crowded bike sheds. Even mounted quite centrally they're vulnerable to knocks and aren't very strong. 

A top tube bag with a clear lid works better for me. Do be sure to get one big enough though.  It will take the battery pack as well and should do a decent job of keeping everything dry (even if you have a waterproof phone you probably want to keep water out of the charging socket).