Multisig Transactions & Application Proposals: Adds metadata to entities

[SovereignAndrey]

Ticket(s):
Request for metadata to be included in subgraph

Related to: AngelProtocolFinance/evm-smart-contracts#228

Explanation of the solution

• Adds metadata bytes to the entities: ApplicationProposal & MultiSigTransaction
• Standardizes Metadata as bytes for input type Charity Applications (previously used string)
• Updates mappings to save passed metadata bytes to the entities

Instructions on making this work

• run yarn or yarn install to install npm dependencies
• run yarn test to verify all tests still pass

[0xNeshi]

We will need to redeploy the changed contracts first and then update their respective addresses (and optionally start blocks) here in subgraph.yaml in order for Subgraph to track those events correctly.

Other than that, all seems good 👍