Avalanche Support

[hrik2001]

Hello, I am working on adding support for avalanche chain. This PR is related to issue humanprotocol/human-protocol#137 Just few things that I want to mention

• For escrow-dashboard, in networkConstants file, I haven't added subgraph api links for the chains as there aren't any yet that are being hosted (I noticed neonslabdev also doesn't have graph api included)
• I have left the field empty for hmt address and escrow factory address for mainnet as it is yet to be deployed
• I have also not added mainnet details in the README due to above mentioned reason

These points were already discussed in discord with @foufrix and @ansaqib, posting it here for reference for others

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[posix4e]

@hrik2001 please sign the cla