fix #518: added switch to allow manual ABI specification
As discussed, I added the possibility to manually specify the ABI to allow interaction with proxied functions (see #518). DAOs are currently not visible on Rinkeby (see #519), I couldn't test it with an actual transaction. Because this is the first time I am working with react, it needs a proper review. Maybe things can be done more elegantly.
For example, the manual ABI specification is necessary to interact with the implementation behind a Gnosis Safe Proxy (see https://gnosis-safe.io/app/rin:0xA7B66EbD4CAE2A7D18cDe96fCA3e362FfCE09601/transactions/0x99c9f2c0a1d87842ab17df93325756ba531a51ff73da7fc21cafc0735a27e536).
We need this to be merged because the AN DAO should be able to remove signers from a Gnosis Safe multi-sig.