Support Gnosis Safe Account Type

Gnosis SAFE provides the ability to control a contract account with multiple keys. Introducing this as an account type in the extension could open up a lot of possibilities.

We'll develop this in three stages:

**1. Single Signer**: SAFE contract controlled by a single key (1 of 1)
* batch transactions are cheaper on gas, happen in perfect order in single tx
* makes lots of extensions possible: account recovery, state channels, limits, allowances, etc
* communicating that your key controls a separate contract
* user needs to publish a contract, which is expensive

**2. Two-Factor**: SAFE contract controlled by two keys.
* Can customize rules around how many keys are required to sign tx
* Can use phone as second factor for tx signing
* Increased security

**3. Multi-Sig**: SAFE contract controlled by an arbitrary number of keys
* Max security
* Usecases for teams, etc
* Have to gather tx signatures off-chain – poses a UX challenge