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/user.go

@@ -409,6 +409,23 @@ func (u *User) PreSave() {
 	}
 }
 
+// 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 (u *User) CreateAt_() float64 {
+	return float64(u.CreateAt)
+}
+
+func (u *User) DeleteAt_() float64 {
+	return float64(u.DeleteAt)
+}
+
+func (u *User) LastPictureUpdateAt() float64 {
+	return float64(u.LastPictureUpdate)
+}
+
 // PreUpdate should be run before updating the user in the db.
 func (u *User) PreUpdate() {
 	u.Username = SanitizeUnicode(u.Username)
@@ -630,6 +647,15 @@ func (u *User) GetCustomStatus() *CustomStatus {
 	return o
 }
 
+func (u *User) CustomStatus() *CustomStatus {
+	var o *CustomStatus
+
+	data := u.Props[UserPropsKeyCustomStatus]
+	_ = json.Unmarshal([]byte(data), &o)
+
+	return o
+}
+
 func (u *User) ClearCustomStatus() {
 	u.MakeNonNil()
 	u.Props[UserPropsKeyCustomStatus] = ""