[NT-1584][NT-1588] Handling rewards and add ons with future start dates

[singhhari]

📲 What

Currently, backers are able to see rewards and add-ons with future start dates and back them. We want to conditionally render these options for selection only when the appropriate time has been reached.

🤔 Why

Currently, rewards and add-ons with start dates in the future are being shown as available for backing. These should be hidden from view until the start date set by the creator.

🛠 How

We're filtering in the RewardAddOnSelectionViewController and RewardCollectionViewController based on a date check.

[codecov]

Codecov Report

Merging #1317 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1317   +/-   ##
=======================================
  Coverage   85.76%   85.77%           
=======================================
  Files        1109     1109           
  Lines       97283    97339   +56     
=======================================
+ Hits        83437    83493   +56     
  Misses      13846    13846           

Impacted Files	Coverage Δ
Library/SharedFunctions.swift	84.23% <100.00%> (+0.53%)	⬆️
Library/SharedFunctionsTests.swift	100.00% <100.00%> (ø)
...ary/ViewModels/RewardAddOnSelectionViewModel.swift	98.46% <100.00%> (-0.02%)	⬇️
...iewModels/RewardAddOnSelectionViewModelTests.swift	99.34% <100.00%> (+0.02%)	⬆️
...ibrary/ViewModels/RewardsCollectionViewModel.swift	99.42% <100.00%> (+<0.01%)	⬆️

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