Bulk multi-select actions, barcode format, batch printing, site name on login

Multi-select (Ctrl/Cmd+click toggle, Shift+click range, right-click the
active selection for bulk actions - same conventions as a native file
manager):
- Print Labels (N) / Export CSV (N) for any mix of individually-picked
  assets and whole folders. Labels/export now take asset_ids/folder_ids
  instead of a single asset_id/folder_id.
- Edit Status..., Check Out..., Duplicate, Move to..., and Delete, bulk
  versions of the existing single-asset actions. Checkout/Move modals
  were generalized to accept an array of ids instead of just one; status
  editing got its own lightweight POST /app/assets/{id}/status endpoint
  (assets.SetStatus) so a bulk status change doesn't need to resend every
  other field the way a full PATCH would.

Labels:
- Choose QR (a /scan/<tag> URL, for a phone camera) or Code128 barcode
  (the bare Asset ID, for a USB scanner typing into the search box) via a
  format selector on the print page. New barcodeDataURI using
  github.com/boombuler/barcode alongside the existing QR generator.
- "Start at #" lets printing resume partway into an already-used sheet
  instead of wasting it on a fresh one.
- Added the Avery 5260 label template (same grid as 5160, rounded
  corners, so it isn't flagged approximate).

Also: the configured Site Name now shows on the login page too, not just
the authenticated navbar.

web/templates/login.html

@@ -3,7 +3,7 @@
 <head>
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
-  <title>Sign in · inventree</title>
+  <title>Sign in · {{siteName}}</title>
   <link rel="stylesheet" href="/static/css/bootstrap.min.css">
   <link rel="stylesheet" href="/static/icons/bootstrap-icons.css">
   <link rel="stylesheet" href="/static/css/app.css">
@@ -14,7 +14,7 @@
       <div class="col-11 col-sm-8 col-md-5 col-lg-4">
         <div class="text-center mb-4">
           <div class="display-6"><i class="bi bi-diagram-3-fill"></i></div>
-          <h1 class="h4 mt-2 mb-0">inventree</h1>
+          <h1 class="h4 mt-2 mb-0">{{siteName}}</h1>
           <div class="text-body-secondary small">IT asset inventory</div>
         </div>
         <div class="card shadow-sm">