refactor: Modify initial loading state for better control of subs section

[julianajlk]

REV-3799.

There was a pre-existing bug that became apparent when subscriptions was introduced and sunset, since we would show the subscriptions section on a conditional - loading's initial state is false, and this caused the component to render before re-rendering in the loading state when that becomes true.

When isB2CSubsEnabled is true:
isLoading is initially false, which makes the page to go into renderOrdersandSubscriptions() (from isLoading ? renderLoading() : renderOrdersandSubscriptions()), which renders <Subscriptions/> and <OrderHistory/> components. Once the fetching of orders and subscriptions happen, isLoading changes to true, and the state change makes the component re-render with or without the subscriptions heading depending on whether the user has subs or not.

In this PR I've changed the loading initial state to start with true for both order-history and subscriptions.
I've also refactored the rendering logic to better accommodate the following scenarios:

1. Subscriptions enabled and user has 0 subscriptions -> render Order History only
2. Subscriptions enabled and user has subscriptions -> render main heading, Subscriptions and Order History.
3. Subscriptions are disabled -> render Order History only

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5cec05d) 61.53% compared to head (5f4e7f8) 63.58%.

Files	Patch %	Lines
...s-and-subscriptions/OrdersAndSubscriptionsPage.jsx	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #356      +/-   ##
==========================================
+ Coverage   61.53%   63.58%   +2.04%     
==========================================
  Files          34       34              
  Lines         325      335      +10     
  Branches       69       74       +5     
==========================================
+ Hits          200      213      +13     
+ Misses        118      115       -3     
  Partials        7        7              

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[NawfalAhmed]

LGTM