Update the visual rendering of the positions

[clemlak]

Positions in Portfolio are ERC1155 tokens and the PositionRenderer contract is responsible for generating their visual representation (including the related metadata). However, this contract was tailored for the NormalStrategy and cannot support other strategies very well.
Now we have a few different options:

• Get rid of the PositionRenderer and don't render visual representations at all (maybe just metadata)
• Improve the PositionRenderer so it can support different strategies
• Let each strategy generate a visual representation by themselves