Shorten the comment field placeholder

The placeholder doubled as a tutorial for mentions and drag-and-drop.
Users discover those on their own, so it now just says "Write a
comment" in both locales.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

i18n/locales/de.json

@@ -100,7 +100,7 @@
   "comments": {
     "none": "Noch keine Kommentare.",
     "loading": "Lade Kommentare…",
-    "placeholder": "Kommentar schreiben… {'@'} zum Erwähnen, Dateien hierher ziehen",
+    "placeholder": "Kommentar schreiben…",
     "submit": "Kommentieren",
     "submitting": "Sendet…",
     "loginToComment": "um zu kommentieren.",

i18n/locales/en.json

@@ -100,7 +100,7 @@
   "comments": {
     "none": "No comments yet.",
     "loading": "Loading comments…",
-    "placeholder": "Write a comment… {'@'} to mention, drop files here",
+    "placeholder": "Write a comment…",
     "submit": "Comment",
     "submitting": "Sending…",
     "loginToComment": "to comment.",