implement custom 404 pages (#81)

solves #56.

- The expected filename is `404.html`, like GitHub Pages
- Each repo/branch can have one `404.html` file at it's root
- If a repo does not have a `pages` branch, the 404.html file from the `pages` repository is used
- You get status code 404 (unless you request /404.html which returns 200)
- The error page is cached

---
close #56

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/81
Reviewed-by: 6543 <6543@noreply.codeberg.org>
Co-authored-by: crystal <crystal@noreply.codeberg.org>
Co-committed-by: crystal <crystal@noreply.codeberg.org>

integration/get_test.go

@@ -49,6 +49,19 @@ func TestGetContent(t *testing.T) {
 	assert.True(t, getSize(resp.Body) > 1000)
 }
 
+func TestGetNotFound(t *testing.T) {
+	log.Printf("== TestGetNotFound ==\n")
+	// test custom not found pages
+	resp, err := getTestHTTPSClient().Get("https://crystal.localhost.mock.directory:4430/pages-404-demo/blah")
+	assert.NoError(t, err)
+	if !assert.EqualValues(t, http.StatusNotFound, resp.StatusCode) {
+		t.FailNow()
+	}
+	assert.EqualValues(t, "text/html; charset=utf-8", resp.Header["Content-Type"][0])
+	assert.EqualValues(t, "37", resp.Header["Content-Length"][0])
+	assert.EqualValues(t, 37, getSize(resp.Body))
+}
+
 func getTestHTTPSClient() *http.Client {
 	cookieJar, _ := cookiejar.New(nil)
 	return &http.Client{