storage: start db migration (badger(hold))

migrating to badger, with the badgerhold frontend. So far, done:
* Announcements (small, for a quick test)
* Discord/Telegram/Matrix/Email

most interaction with badgerhold is done through the standard
Get<x>/Get<x>Key/Set<x>Key/Delete<x>Key. UserExists functions have been
added for email and matrix, and those and the original ones now use a
query against the database rather than sifting through every record.
I've tagged these searched fields as "index" for badgerhold, although this
definitely isn't used yet, and i'm not entirely sure if it'll be useful.

migrateToBadger is now in migrations.go, and a temporary config key
"migrated_to_badger" has been added, although it isn't being used yet,
migration is just running every time during development.

matrix.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/gomarkdown/markdown"
+	"github.com/timshannon/badgerhold/v4"
 	"maunium.net/go/mautrix"
 	"maunium.net/go/mautrix/event"
 	"maunium.net/go/mautrix/id"
@@ -31,11 +32,12 @@ type UnverifiedUser struct {
 }
 
 type MatrixUser struct {
-	RoomID    string
-	Encrypted bool
-	UserID    string
-	Lang      string
-	Contact   bool
+	RoomID     string
+	Encrypted  bool
+	UserID     string
+	Lang       string
+	Contact    bool
+	JellyfinID string `badgerhold:"key"`
 }
 
 var matrixFilter = mautrix.Filter{
@@ -268,6 +270,12 @@ func (d *MatrixDaemon) Send(message *Message, users ...MatrixUser) (err error) {
 	return
 }
 
+// UserExists returns whether or not a user with the given User ID exists.
+func (d *MatrixDaemon) UserExists(userID string) bool {
+	c, err := d.app.storage.db.Count(&MatrixUser{}, badgerhold.Where("UserID").Eq(userID))
+	return err != nil || c > 0
+}
+
 // User enters ID on sign-up, a PIN is sent to them. They enter it on sign-up.
 
 // Message the user first, to avoid E2EE by default