Add team member invite by email (#20307)

Allows to add (not registered) team members by email.

related #5353

Invite by mail:

![grafik](https://user-images.githubusercontent.com/1666336/178154779-adcc547f-c0b7-4a2a-a131-4e41a3d9d3ad.png)

Pending invitations:

![grafik](https://user-images.githubusercontent.com/1666336/178154882-9d739bb8-2b04-46c1-a025-c1f4be26af98.png)

Email:

![grafik](https://user-images.githubusercontent.com/1666336/178164716-f2f90893-7ba6-4a5e-a3db-42538a660258.png)

Join form:

![grafik](https://user-images.githubusercontent.com/1666336/178154840-aaab983a-d922-4414-b01a-9b1a19c5cef7.png)

Co-authored-by: Jack Hay <jjphay@gmail.com>

models/organization/team_invite_test.go

@@ -0,0 +1,49 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package organization_test
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/organization"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestTeamInvite(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	team := unittest.AssertExistsAndLoadBean(t, &organization.Team{ID: 2})
+
+	t.Run("MailExistsInTeam", func(t *testing.T) {
+		user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+
+		// user 2 already added to team 2, should result in error
+		_, err := organization.CreateTeamInvite(db.DefaultContext, user2, team, user2.Email)
+		assert.Error(t, err)
+	})
+
+	t.Run("CreateAndRemove", func(t *testing.T) {
+		user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})
+
+		invite, err := organization.CreateTeamInvite(db.DefaultContext, user1, team, "user3@example.com")
+		assert.NotNil(t, invite)
+		assert.NoError(t, err)
+
+		// Shouldn't allow duplicate invite
+		_, err = organization.CreateTeamInvite(db.DefaultContext, user1, team, "user3@example.com")
+		assert.Error(t, err)
+
+		// should remove invite
+		assert.NoError(t, organization.RemoveInviteByID(db.DefaultContext, invite.ID, invite.TeamID))
+
+		// invite should not exist
+		_, err = organization.GetInviteByToken(db.DefaultContext, invite.Token)
+		assert.Error(t, err)
+	})
+}