GraphQL (part 2): Model changes and resolvers (#19486)

This PR adds the necessary resolvers to support
the graphQL front-end. Some additional methods
need to be added to the model structs to support
this. Comments have been made to clarify their purpose.

There is a slight duplication of code in the api layer.
But eventually that's going to go away when we move
away from the REST API entirely.

The schema is in api4/schema.graphqls and gets compiled
into the binary as an asset.

The GraphiQL editor url is at GET /api/v5/graphql.

Everything is behind the feature flag MM_FEATUREFLAGS_GRAPHQL.

```release-note
NONE
```

model/status.go

@@ -34,6 +34,19 @@ func (s *Status) ToJSON() ([]byte, error) {
 	return json.Marshal(sCopy)
 }
 
+// The following are some GraphQL methods necessary to return the
+// data in float64 type. The spec doesn't support 64 bit integers,
+// so we have to pass the data in float64. The _ at the end is
+// a hack to keep the attribute name same in GraphQL schema.
+
+func (s *Status) LastActivityAt_() float64 {
+	return float64(s.LastActivityAt)
+}
+
+func (s *Status) DNDEndTime_() float64 {
+	return float64(s.DNDEndTime)
+}
+
 func StatusListToJSON(u []*Status) ([]byte, error) {
 	list := make([]Status, len(u))
 	for i, s := range u {