Add default board to new projects, remove uncategorized pseudo-board (#29874)

On creation of an empty project (no template) a default board will be
created instead of falling back to the uneditable pseudo-board.

Every project now has to have exactly one default boards. As a
consequence, you cannot unset a board as default, instead you have to
set another board as default. Existing projects will be modified using a
cron job, additionally this check will run every midnight by default.

Deleting the default board is not allowed, you have to set another board
as default to do it.

Fixes #29873
Fixes #14679 along the way
Fixes #29853

Co-authored-by: delvh <dev.lh@web.de>
(cherry picked from commit e5160185ed65fd1c2bcb2fc7dc7e0b5514ddb299)

Conflicts:
	options/locale/locale_en-US.ini
	trivial conflict because Forgejo strings do not have
	surrounding double quotes

models/project/board_test.go

@@ -0,0 +1,40 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package project
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetDefaultBoard(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	projectWithoutDefault, err := GetProjectByID(db.DefaultContext, 5)
+	assert.NoError(t, err)
+
+	// check if default board was added
+	board, err := projectWithoutDefault.getDefaultBoard(db.DefaultContext)
+	assert.NoError(t, err)
+	assert.Equal(t, int64(5), board.ProjectID)
+	assert.Equal(t, "Uncategorized", board.Title)
+
+	projectWithMultipleDefaults, err := GetProjectByID(db.DefaultContext, 6)
+	assert.NoError(t, err)
+
+	// check if multiple defaults were removed
+	board, err = projectWithMultipleDefaults.getDefaultBoard(db.DefaultContext)
+	assert.NoError(t, err)
+	assert.Equal(t, int64(6), board.ProjectID)
+	assert.Equal(t, int64(8), board.ID)
+
+	board, err = GetBoard(db.DefaultContext, 9)
+	assert.NoError(t, err)
+	assert.Equal(t, int64(6), board.ProjectID)
+	assert.False(t, board.Default)
+}