Set safeTxGas=0 for rejections

[tschubotz]

What is this feature about? (1 sentence)

This sets safeTxGas to 0 for on-chain rejections in order to be in line with mobile apps and client gateway.

Why is it needed? What is the value? For whom do we build it?

• On-chain rejections are Safe txs to the Safe itself with data set to 0x/null
• Currently, we still estimates safeTxGas properly and uses that value.
• That means that currently many different on-chain rejections can be created as long as they use different safeTxGas value.
• With "Proper rejections" we wanted to streamline that by setting all params, including safeTxGas to 0x/null/0.
• That way, only 1 type of on-chain rejection can be created for a given nonce (Since the safeTxHash would be the same always).
• Currently mobile and web create different types of on-chain rejections which is a challenge for the client gateway and UX.

High-level overview of the feature

• When creating rejections, set safeTxGas to 0 instead of doing gas estimation for it.