changes

* move error-related code for groups to its own file

* update group avatar logic

remove unused/duplicate logic

* update `FindGroupsOptions.ToConds()`

allow passing `-1` as the `ParentGroupID`, meaning "find matching groups regardless of the parent group id"

* add `DedupeBy` function to container module

this removes duplicate items from a slice using a custom function

* add `SliceMap` util

works like javascripts's `Array.prototoype.map`, taking in a slice and transforming each element with the provided function

* add group service

functions included so far:
- avatar uploading/deletion
- group deletion
- group creation
- group moving (including moving item inside a group)
- group update
- team management
  - add team
  - remove team
  - update team permissions
  - recalculating team access (in event of group move)
- group searching (only used in frontend/web components for now)

models/group/errors.go

@@ -0,0 +1,41 @@
+package group
+
+import (
+	"errors"
+	"fmt"
+
+	"code.gitea.io/gitea/modules/util"
+)
+
+type ErrGroupNotExist struct {
+	ID int64
+}
+
+// IsErrGroupNotExist checks if an error is a ErrCommentNotExist.
+func IsErrGroupNotExist(err error) bool {
+	var errGroupNotExist ErrGroupNotExist
+	ok := errors.As(err, &errGroupNotExist)
+	return ok
+}
+
+func (err ErrGroupNotExist) Error() string {
+	return fmt.Sprintf("group does not exist [id: %d]", err.ID)
+}
+
+func (err ErrGroupNotExist) Unwrap() error {
+	return util.ErrNotExist
+}
+
+type ErrGroupTooDeep struct {
+	ID int64
+}
+
+func IsErrGroupTooDeep(err error) bool {
+	var errGroupTooDeep ErrGroupTooDeep
+	ok := errors.As(err, &errGroupTooDeep)
+	return ok
+}
+
+func (err ErrGroupTooDeep) Error() string {
+	return fmt.Sprintf("group has reached or exceeded the subgroup nesting limit [id: %d]", err.ID)
+}