lib/api: Save session & CSRF tokens to database, add option to stay logged in (fixes #9151) (#9284)

This adds a "token manager" which handles storing and checking expired
tokens, used for both sessions and CSRF tokens. It removes the old,
corresponding functionality for CSRFs which saved things in a file. The
result is less crap in the state directory, and active login sessions
now survive a Syncthing restart (this really annoyed me).

It also adds a boolean on login to create a longer-lived session cookie,
which is now possible and useful. Thus we can remain logged in over
browser restarts, which was also annoying... :)

<img width="1001" alt="Screenshot 2023-12-12 at 09 56 34"
src="https://github.com/syncthing/syncthing/assets/125426/55cb20c8-78fc-453e-825d-655b94c8623b">

Best viewed with whitespace-insensitive diff, as a bunch of the auth
functions became methods instead of closures which changed indentation.

lib/api/api_auth_test.go

@@ -8,8 +8,12 @@ package api
 
 import (
 	"testing"
+	"time"
 
 	"github.com/syncthing/syncthing/lib/config"
+	"github.com/syncthing/syncthing/lib/db"
+	"github.com/syncthing/syncthing/lib/db/backend"
+	"github.com/syncthing/syncthing/lib/events"
 )
 
 var guiCfg config.GUIConfiguration
@@ -110,3 +114,76 @@ func TestEscapeForLDAPDN(t *testing.T) {
 		}
 	}
 }
+
+type mockClock struct {
+	now time.Time
+}
+
+func (c *mockClock) Now() time.Time {
+	c.now = c.now.Add(1) // time always ticks by at least 1 ns
+	return c.now
+}
+
+func (c *mockClock) wind(t time.Duration) {
+	c.now = c.now.Add(t)
+}
+
+func TestTokenManager(t *testing.T) {
+	t.Parallel()
+
+	mdb, _ := db.NewLowlevel(backend.OpenMemory(), events.NoopLogger)
+	kdb := db.NewNamespacedKV(mdb, "test")
+	clock := &mockClock{now: time.Now()}
+
+	// Token manager keeps up to three tokens with a validity time of 24 hours.
+	tm := newTokenManager("testTokens", kdb, 24*time.Hour, 3)
+	tm.timeNow = clock.Now
+
+	// Create three tokens
+	t0 := tm.New()
+	t1 := tm.New()
+	t2 := tm.New()
+
+	// Check that the tokens are valid
+	if !tm.Check(t0) {
+		t.Errorf("token %q should be valid", t0)
+	}
+	if !tm.Check(t1) {
+		t.Errorf("token %q should be valid", t1)
+	}
+	if !tm.Check(t2) {
+		t.Errorf("token %q should be valid", t2)
+	}
+
+	// Create a fourth token
+	t3 := tm.New()
+	// It should be valid
+	if !tm.Check(t3) {
+		t.Errorf("token %q should be valid", t3)
+	}
+	// But the first token should have been removed
+	if tm.Check(t0) {
+		t.Errorf("token %q should be invalid", t0)
+	}
+
+	// Wind the clock by 12 hours
+	clock.wind(12 * time.Hour)
+	// The second token should still be valid (and checking it will give it more life)
+	if !tm.Check(t1) {
+		t.Errorf("token %q should be valid", t1)
+	}
+
+	// Wind the clock by 12 hours
+	clock.wind(12 * time.Hour)
+	// The second token should still be valid
+	if !tm.Check(t1) {
+		t.Errorf("token %q should be valid", t1)
+	}
+	// But the third and fourth tokens should have expired
+	if tm.Check(t2) {
+		t.Errorf("token %q should be invalid", t2)
+	}
+	if tm.Check(t3) {
+		t.Errorf("token %q should be invalid", t3)
+	}
+}