Refactor: GymDetailViewController Sections

[kevinchan159]

Currently, inside GymDetailViewController we define an enum Section follows:

    // MARK: - Private classes/enums
    private struct Section {
        var items: [ItemType]
    }

and then define var sections: [Section] to just be:

self.sections = [
  Section(items: [.hours, .busyTimes, .facilities, .classes([])])
]

This means that what we end up with is just one giant section with 4 cells.

We should change this so that we have 4 sections, with one cell in each. This is will help reduce with the increasingly nested view hierarchy we have