v0.7.4: Merge pull request #1963 from fetchai/develop

• Replaces error skill handler usage with built in handler
• Extends MultiAgentManager to support persistence between runs
• Replaces usage of Ropsten testnet with Ganache
• Adds support for symlink creation during scaffold and add
• Makes contract interface loading extensible
• Adds support for PEP561
• Adds integration tests for launcher command
• Adds support for storage of unique page address in SOEF
• Fixes publish command bug on Windows
• Refactors constants usage throughout
• Adds support for profiling on aea run
• Multiple stability improvements to core async modules
• Multiple docs updates based on user feedback
• Multiple additional tests and test stability fixes