Update API for improved compatibility with Redux

[nowyasimi]

Some changes can be made to the API to improve general code readability and performance. This will make it easier to retrieve and pass information to the relevant components. Changes to make:

• Convert dictionaries of sections to lists of sections
• Extract extra sections that are independent of their main section. That means adding a independent_extra_section_lists attribute for each course in the API:

{
    "cache_time": ...,
    "sections": ...,
    "independent_extra_section_lists": ...
}