Add "approve" and "reject" user methods (#1007)

Fixes #1006

Author: jmini

src/main/java/org/gitlab4j/api/UserApi.java

@@ -169,6 +169,36 @@ public Stream<User> getActiveUsersStream() throws GitLabApiException {
         return (getActiveUsers(getDefaultPerPage()).stream());
     }
 
+    /**
+     * Approves the specified user. Available only for admin.
+     *
+     * <pre><code>GitLab Endpoint: POST /users/:id/approve</code></pre>
+     *
+     * @param userId the ID of the user to approve
+     * @throws GitLabApiException if any exception occurs
+     */
+    public void approveUser(Long userId) throws GitLabApiException {
+        if (userId == null) {
+            throw new RuntimeException("userId cannot be null");
+        }
+        post(Response.Status.CREATED, (Form) null, "users", userId, "approve");
+    }
+
+    /**
+     * Rejects specified user that is pending approval. Available only for administrators.
+     *
+     * <pre><code>GitLab Endpoint: POST /users/:id/reject</code></pre>
+     *
+     * @param userId the ID of the user to reject
+     * @throws GitLabApiException if any exception occurs
+     */
+    public void rejectUser(Long userId) throws GitLabApiException {
+        if (userId == null) {
+            throw new RuntimeException("userId cannot be null");
+        }
+        post(Response.Status.OK, (Form) null, "users", userId, "reject");
+    }
+
     /**
      * Blocks the specified user. Available only for admin.
      *