Show a more attractive icon for custom RPC networks

[danfinlay]

Today, custom RPCs show a grey question mark.

Users chose this RPC, so it should not be in question (unless it's offline or something).

We should choose something more attractive to show. Even just a black circle. Maybe a tiny jazzicon derived from the RPC.

[bschorchit]

We have a ticket to do so here #16679 (already on PS team backlog) and a slack thread about it here :)