feat: use cobra to provide subcommands, move sources to lib (#506)

- Use cobra in order to provide subcommands `serve` and `db`.
  - Subdir `cmd` is removed.
  - Subdir `cli` is created, which is a standard cobra structure.
- Sources related to the core are moved to subdir `lib`.
- #497 and #504 are merged.
- Deprecated flags are added. See https://github.com/filebrowser/filebrowser/pull/497#discussion_r209428120.
- [`viper.BindPFlags`](https://godoc.org/github.com/spf13/viper#BindPFlags) is used in order to reduce the verbosity in `serve.go`.

lib/bolt/users.go

@@ -0,0 +1,90 @@
+package bolt
+
+import (
+	"reflect"
+
+	"github.com/asdine/storm"
+	fb "github.com/filebrowser/filebrowser/lib"
+)
+
+// UsersStore is a users store.
+type UsersStore struct {
+	DB *storm.DB
+}
+
+// Get gets a user with a certain id from the database.
+func (u UsersStore) Get(id int, builder fb.FSBuilder) (*fb.User, error) {
+	var us fb.User
+	err := u.DB.One("ID", id, &us)
+	if err == storm.ErrNotFound {
+		return nil, fb.ErrNotExist
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	us.FileSystem = builder(us.Scope)
+	return &us, nil
+}
+
+// GetByUsername gets a user with a certain username from the database.
+func (u UsersStore) GetByUsername(username string, builder fb.FSBuilder) (*fb.User, error) {
+	var us fb.User
+	err := u.DB.One("Username", username, &us)
+	if err == storm.ErrNotFound {
+		return nil, fb.ErrNotExist
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	us.FileSystem = builder(us.Scope)
+	return &us, nil
+}
+
+// Gets gets all the users from the database.
+func (u UsersStore) Gets(builder fb.FSBuilder) ([]*fb.User, error) {
+	var us []*fb.User
+	err := u.DB.All(&us)
+	if err == storm.ErrNotFound {
+		return nil, fb.ErrNotExist
+	}
+
+	if err != nil {
+		return us, err
+	}
+
+	for _, user := range us {
+		user.FileSystem = builder(user.Scope)
+	}
+
+	return us, err
+}
+
+// Update updates the whole user object or only certain fields.
+func (u UsersStore) Update(us *fb.User, fields ...string) error {
+	if len(fields) == 0 {
+		return u.Save(us)
+	}
+
+	for _, field := range fields {
+		val := reflect.ValueOf(us).Elem().FieldByName(field).Interface()
+		if err := u.DB.UpdateField(us, field, val); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// Save saves a user to the database.
+func (u UsersStore) Save(us *fb.User) error {
+	return u.DB.Save(us)
+}
+
+// Delete deletes a user from the database.
+func (u UsersStore) Delete(id int) error {
+	return u.DB.DeleteStruct(&fb.User{ID: id})
+}