Community pool spend proposal (#4329)

Implement the "CommunityPoolSpendProposal" as described in Cosmos Hub proposal 7.

Also a useful test of Git flow for merging features passed in governance proposals.

x/params/simulation/msgs.go

@@ -5,6 +5,7 @@ import (
 	"math/rand"
 	"time"
 
+	"github.com/cosmos/cosmos-sdk/baseapp"
 	sdk "github.com/cosmos/cosmos-sdk/types"
 	"github.com/cosmos/cosmos-sdk/x/gov"
 	"github.com/cosmos/cosmos-sdk/x/params"
@@ -109,7 +110,7 @@ var paramChangePool = []simParamChange{
 // SimulateParamChangeProposalContent returns random parameter change content.
 // It will generate a ParameterChangeProposal object with anywhere between 1 and
 // 3 parameter changes all of which have random, but valid values.
-func SimulateParamChangeProposalContent(r *rand.Rand) gov.Content {
+func SimulateParamChangeProposalContent(r *rand.Rand, _ *baseapp.BaseApp, _ sdk.Context, _ []simulation.Account) gov.Content {
 	numChanges := simulation.RandIntBetween(r, 1, len(paramChangePool)/2)
 	paramChanges := make([]params.ParamChange, numChanges, numChanges)
 	paramChangesKeys := make(map[string]struct{})