Add pagination to comments to prevent browser crash

Posts with 1000+ comments (like ecency.waves posts with 1962 comments)
caused Chrome to crash with "Aw, Snap!" due to rendering all comments
at once.

Changes:
- Limit initial display to 50 root-level comments
- Add "Show more comments" button to load 50 more at a time
- Only applies to root-level (depth 1) comments, nested threads
  still fully expand
- Added translation key for all locales

Fixes #720