Server improvements

• add 400 checking middleware. If passed task ID is invalid return 400 status code.
• add 404 checking middleware. Task is needed by some methods (GET, PUT, DELETE), to reduce repeated code checkExistence middleware is added. It tries to find task by ID, and if it doesn't - 404 status code is returned.
• change handlers URLs to follow REST-spec. Method names (PUT, DELETE) are self describing, it doesn't required to repeat them into the URL.
• change tasks store data structure from a list to a map. Because of huge-huge lists of tasks an operation on task could take a lot of time, finding an item in the array is O(n). That's why the list is changed to the map (item finding time is O(1)). It seems to be a huge improvement.
• add checkTitle middleware to check passed params in request body. If there is no title param (which is required I belive) then 400 status code is returned.
• change response status codes and response body due to the REST.

Writing tests

• add unit tests for the tasks reducer (client side)
• add integration tests for the Drawer and TasksList components.

Socket

• add socket connection between the client and the server
• add broadcasting events about creation, changing and removing test
• subscribe to the socket events and respond correctly by changin the client state

Responsiveness

• add breakpoints sass-mixin
• add masonry-like view of tasks at devices with min width > 768px

Tradeoffs

• I know that it should be a mock server to test API calls, but for simplicity I've decided to keep my main server for that purpose.
• I've added only one test for the server side just to show how it works
• there should be a filtering function to remove self-sended socket events. Eg. when the client is changing the task it will register external-change event as well. So, it should be a client ID into the socket event and the client should filter self-maded events.
• the desktop view maybe not so ux-friendly, I've just shown a way to create responsive layouts