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/team_member.go

@@ -125,3 +125,9 @@ func (o *TeamMember) PreUpdate() {
 func (o *TeamMember) GetRoles() []string {
 	return strings.Fields(o.Roles)
 }
+
+// DeleteAt_ returns the deleteAt value in float64. This is necessary to work
+// with GraphQL since it doesn't support 64 bit integers.
+func (o *TeamMember) DeleteAt_() float64 {
+	return float64(o.DeleteAt)
+}