Move convert package to services (#22264)

Addition to #22256

The `convert` package relies heavily on different models which is
[disallowed by our definition of
modules](https://github.com/go-gitea/gitea/blob/main/CONTRIBUTING.md#design-guideline).
This helps to prevent possible import cycles.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/convert/utils_test.go

@@ -0,0 +1,39 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package convert
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func TestToCorrectPageSize(t *testing.T) {
+	assert.EqualValues(t, 30, ToCorrectPageSize(0))
+	assert.EqualValues(t, 30, ToCorrectPageSize(-10))
+	assert.EqualValues(t, 20, ToCorrectPageSize(20))
+	assert.EqualValues(t, 50, ToCorrectPageSize(100))
+}
+
+func TestToGitServiceType(t *testing.T) {
+	tc := []struct {
+		typ  string
+		enum int
+	}{{
+		typ: "github", enum: 2,
+	}, {
+		typ: "gitea", enum: 3,
+	}, {
+		typ: "gitlab", enum: 4,
+	}, {
+		typ: "gogs", enum: 5,
+	}, {
+		typ: "trash", enum: 1,
+	}}
+	for _, test := range tc {
+		assert.EqualValues(t, test.enum, ToGitServiceType(test.typ))
+	}
+}