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
```

api4/schema.graphqls

@@ -0,0 +1,157 @@
+schema {
+  query: Query
+}
+
+type Query {
+	user(id: String!): User
+	config(): StringMap!
+	license(): StringMap!
+	teamMembers(userId: String!,
+		teamId: String = ""): [TeamMember]!
+	channels(userId: String!,
+		teamId: String = "",
+		includeDeleted: Boolean = false,
+		lastDeleteAt: Float = 0,
+		lastUpdateAt: Float = 0,
+		first: Int = 60,
+		after: String = ""): [Channel]!
+	channelsLeft(userId: String!,
+		since: Float!): [String!]!
+	channelMembers(userId: String!,
+		channelId: String = "",
+		first: Int = 60,
+		after: String = "",
+		lastUpdateAt: Float = 0): [ChannelMember]!
+}
+
+scalar ChannelType
+
+scalar SidebarCategoryType
+
+scalar SidebarCategorySorting
+
+scalar StringMap
+
+scalar Time
+
+type Channel {
+	id           : String!
+	createAt    : Float!
+	updateAt    : Float!
+	deleteAt    : Float!
+	type : ChannelType!
+	displayName: String!
+	prettyDisplayName: String!
+	name: String!
+	header: String!
+	purpose: String!
+	creatorId: String!
+	schemeId: String
+	team: Team
+	cursor: String
+}
+
+type ChannelMember {
+	channel          : Channel
+	user             : User
+	roles            : [Role]!
+	lastViewedAt     : Float!
+	msgCount         : Float!
+	mentionCount     : Float!
+	mentionCountRoot : Float!
+	notifyProps      : StringMap!
+	lastUpdateAt     : Float!
+	schemeGuest      : Boolean!
+	schemeUser       : Boolean!
+	schemeAdmin      : Boolean!
+	explicitRoles    : String!
+	cursor: String
+}
+
+type User {
+	id: String!
+	username: String!
+	email: String!
+	firstName: String!
+	lastName: String!
+	nickname: String!
+	isBot: Boolean!
+	isGuest: Boolean!
+	isSystemAdmin: Boolean!
+	createAt: Float!
+	deleteAt: Float!
+	authService: String!
+	customStatus: CustomStatus
+	status: Status
+	props: StringMap!
+	notifyProps: StringMap!
+	lastPictureUpdateAt: Float!
+	locale: String!
+	timezone: StringMap!
+	position: String!
+	roles: [Role]!
+	preferences: [Preference!]!
+}
+
+type CustomStatus {
+	emoji: String!
+	text: String!
+	duration: String!
+	expiresAt: Time!
+}
+
+type Status {
+	status: String!
+	manual: Boolean!
+	lastActivityAt: Float!
+	activeChannel: String!
+	dndEndTime: Float!
+}
+
+type Role {
+	id: String!
+	name: String!
+	permissions: [String!]!
+	schemeManaged: Boolean!
+	builtIn: Boolean!
+}
+
+type Preference {
+	userId: String!
+	category: String!
+	name: String!
+	value: String!
+}
+
+type Team {
+	id: String!
+	displayName    :    String!
+	name           :    String!
+	description    :    String!
+	email          :    String!
+	type           :    String!
+	companyName    :    String!
+	allowedDomains :    String!
+	inviteId       :    String!
+}
+
+type TeamMember {
+	team: Team
+	user: User
+	roles: [Role]!
+	deleteAt: Float!
+	schemeGuest: Boolean!
+	schemeUser: Boolean!
+	schemeAdmin: Boolean!
+	sidebarCategories: [SidebarCategory]!
+}
+
+type SidebarCategory {
+	id: String!
+	sorting: SidebarCategorySorting!
+	type: SidebarCategoryType!
+	displayName: String!
+	muted: Boolean!
+	collapsed: Boolean!
+	channelIds: [String!]!
+}