Split code into modules

This is a massive commit that restructures the code into modules:

db/
    All functions related to modifying the Database

types/
    All type definitions and methods that can be exclusivly used on
    these types without dependencies

policy/
    All Policy related code, now without dependencies on the Database.

policy/matcher/
    Dedicated code to match machines in a list of FilterRules

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

hscontrol/types/users.go

@@ -0,0 +1,55 @@
+package types
+
+import (
+	"strconv"
+	"time"
+
+	v1 "github.com/juanfont/headscale/gen/go/headscale/v1"
+	"github.com/juanfont/headscale/hscontrol/util"
+	"google.golang.org/protobuf/types/known/timestamppb"
+	"gorm.io/gorm"
+	"tailscale.com/tailcfg"
+)
+
+// User is the way Headscale implements the concept of users in Tailscale
+//
+// At the end of the day, users in Tailscale are some kind of 'bubbles' or users
+// that contain our machines.
+type User struct {
+	gorm.Model
+	Name string `gorm:"unique"`
+}
+
+func (n *User) TailscaleUser() *tailcfg.User {
+	user := tailcfg.User{
+		ID:            tailcfg.UserID(n.ID),
+		LoginName:     n.Name,
+		DisplayName:   n.Name,
+		ProfilePicURL: "",
+		Domain:        "headscale.net",
+		Logins:        []tailcfg.LoginID{},
+		Created:       time.Time{},
+	}
+
+	return &user
+}
+
+func (n *User) TailscaleLogin() *tailcfg.Login {
+	login := tailcfg.Login{
+		ID:            tailcfg.LoginID(n.ID),
+		LoginName:     n.Name,
+		DisplayName:   n.Name,
+		ProfilePicURL: "",
+		Domain:        "headscale.net",
+	}
+
+	return &login
+}
+
+func (n *User) Proto() *v1.User {
+	return &v1.User{
+		Id:        strconv.FormatUint(uint64(n.ID), util.Base10),
+		Name:      n.Name,
+		CreatedAt: timestamppb.New(n.CreatedAt),
+	}
+}