Add bounty support to polkassembly

[niklabh]

Treasury pallet is supporting bounties now. Create a new post in polkassembly on proposeBounty call. Also track approveBounty call.

• [ ] Track bounty in node watcher
• [x] Create on chain bounty column in on chain link
• [x] Listen for new bounties in chain db watcher
• [x] Create posts when new bounties are added
• [ ] Update types in chain db open server.