Make sidebar better

[danmaninc]

There is duplicating code for the sidebar. Due to that, user renders two sidebars at the same time: the first for mobile, and the second for the desktop. At least one (always) of them is hidden.

This issue aims to remove duplicating code to possibly improve performance and page loading speed.

The following tasks may be completed:

• [x] #39
• [ ] Sidebar hide button on mobile devices (?)
• [x] #132
• [ ] Create nested subsections in the sidebar (?)

[ArtemSBulgakov]

Возможно, сделать кнопку скрытия сайдбара на телефоне; на телефоне добавить анимацию открытия (#39); убрать дублирование элементов в React.

[ArtemSBulgakov]

Возможно, сделать возможность создания вложенностей: внутри Schedule есть Sports, Academic courses и т.д.