[BUG] Orders|Trustlines|Balances: Allow ledger hash query parameter (RLJS-137)

This only checks if it's a valid hex string. A valid hash would have 32 characters. You can expand this regex to check for length 32 or use ripple-lib's `Uint256.is_valid()`. Either is fine, but I would put it in a utils function `isValidHash()`