Implement the "CommunityPoolSpendProposal" as described in Cosmos Hub proposal 7.
Also a useful test of Git flow for merging features passed in governance proposals.
Gaia is removed from cosmos-sdk repository.
Few changes were required to make sure no packages depend on gaia subpackages.
CI config is amended accordingly.
Unnecessary targets are removed from Makefile.
Simulations run through a lightweight version of gaia renamed to simapp.
Closes: #4104