Fix Solidity linter errors

contracts/drafts/meta-tx/GSNContext.sol

@@ -42,12 +42,14 @@ contract GSNContext is Context {
         // require bit shifting. We therefore subtract 12 from the read index so the address lands on the lower 20
         // bytes. This can always be done due to the 32-byte prefix.
 
-        // The final memory read index is msg.data.length - 20 + 32 - 12 = msg.data.length
+        // The final memory read index is msg.data.length - 20 + 32 - 12 = msg.data.length. Using inline assembly is the
+        // easiest/most-efficient way to perform this operation.
 
         // These fields are not accessible from assembly
         bytes memory array = msg.data;
         uint256 index = msg.data.length;
 
+        // solhint-disable-next-line no-inline-assembly
         assembly {
             // Load the 32 bytes word from memory with the address on the lower 20 bytes, and mask those.
             result := and(mload(add(array, index)), 0xffffffffffffffffffffffffffffffffffffffff)

contracts/drafts/meta-tx/GSNRecipient.sol

@@ -8,14 +8,14 @@ import "./GSNContext.sol";
  * GSN support. Derived contracts must implement all interface methods.
  */
 contract GSNRecipient is IRelayRecipient, GSNContext {
-    uint256 constant private RelayedCallAccepted = 0;
-    uint256 constant private RelayedCallRejected = 11;
+    uint256 constant private RELAYED_CALL_ACCEPTED = 0;
+    uint256 constant private RELAYED_CALL_REJECTED = 11;
 
     function _acceptRelayedCall() internal pure returns (uint256) {
-        return RelayedCallAccepted;
+        return RELAYED_CALL_ACCEPTED;
     }
 
     function _declineRelayedCall(uint256 errorCode) internal pure returns (uint256) {
-        return RelayedCallRejected + errorCode;
+        return RELAYED_CALL_REJECTED + errorCode;
     }
 }

contracts/drafts/meta-tx/GSNRecipientERC20BuiltInCharge.sol

@@ -30,7 +30,9 @@ contract GSNRecipientERC20BuiltInCharge is GSNRecipientERC20Charge {
 contract __unstable__ERC20PrimaryAdmin is ERC20, ERC20Detailed, Secondary {
     uint256 private constant UINT256_MAX = 2**256 - 1;
 
-    constructor(string memory name, string memory symbol, uint8 decimals) ERC20Detailed(name, symbol, decimals) public { }
+    constructor(string memory name, string memory symbol, uint8 decimals) public ERC20Detailed(name, symbol, decimals) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
 
     // The primary account (GSNRecipientERC20BuiltInCharge) can mint tokens
     function mint(address account, uint256 amount) public onlyPrimary {

contracts/drafts/meta-tx/GSNRecipientSignedData.sol

@@ -49,7 +49,11 @@ contract GSNRecipientSignedData is GSNRecipient {
         }
     }
 
-    function preRelayedCall(bytes calldata) external returns (bytes32) { }
+    function preRelayedCall(bytes calldata) external returns (bytes32) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
 
-    function postRelayedCall(bytes calldata, bool, uint256, bytes32) external { }
+    function postRelayedCall(bytes calldata, bool, uint256, bytes32) external {
+        // solhint-disable-previous-line no-empty-blocks
+    }
 }

contracts/mocks/GSNContextMock.sol

@@ -24,7 +24,11 @@ contract GSNContextMock is ContextMock, GSNContext, IRelayRecipient {
         return (0, "");
     }
 
-    function preRelayedCall(bytes calldata) external returns (bytes32) {}
+    function preRelayedCall(bytes calldata) external returns (bytes32) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
 
-    function postRelayedCall(bytes calldata, bool, uint256, bytes32) external {}
+    function postRelayedCall(bytes calldata, bool, uint256, bytes32) external {
+        // solhint-disable-previous-line no-empty-blocks
+    }
 }

contracts/mocks/GSNRecipientSignedDataMock.sol

@@ -3,7 +3,9 @@ pragma solidity ^0.5.0;
 import "../drafts/meta-tx/GSNRecipientSignedData.sol";
 
 contract GSNRecipientSignedDataMock is GSNRecipientSignedData {
-    constructor(address trustedSigner) public GSNRecipientSignedData(trustedSigner) { }
+    constructor(address trustedSigner) public GSNRecipientSignedData(trustedSigner) {
+        // solhint-disable-previous-line no-empty-blocks
+    }
 
     event MockFunctionCalled();