Ensure consistent naming of title fields (#528)

Remove task text and namespace name in migration

Fix lint

Add migration for namespace title

Fix renaming namespace name to title

Rename namespace name field to title

Drop text column at the end of the migration

Add migration for task text to title

Rename task text to title

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/528

pkg/modules/migration/migrator.go

@@ -30,7 +30,7 @@ type Migrator interface {
 	// The use case for this are Oauth flows, where the server token should remain hidden and not
 	// known to the frontend.
 	AuthURL() string
-	// Name holds the name of the migration.
+	// Title holds the name of the migration.
 	// This is used to show the name to users and to keep track of users who already migrated.
 	Name() string
 }