Refactor: GymDetailViewController Sections

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