1.40.8 build fixes

[bleetube]

I was exploring the idea of making a custom campaign for a OST remapping project. Ostensibly it's just a map pack, but thought it might be fun to make a campaign where the player has to pass each OST in succession, or something like that.

First commit has minimal changes to build against BS 1.40.8 (and presumably any 1.40.x release). I chose this version since I still use the 1.40.x release (no PlaylistManager updates for newer releases yet) so this is the easiest version for me to test with right now.

I still get some harmless compiler warnings that I can clean up if you're open to having me propose more changes.

dotnet build "CustomCampaigns/CustomCampaigns.csproj" -c Debug \
  -p:BeatSaberDir="/home/pleb/.local/share/BSManager/BSInstances/1.40.8"
Restore complete (0.2s)
  CustomCampaigns succeeded with 6 warning(s) (0.4s) → CustomCampaigns/bin/Debug/net48/Custom Campaigns.dll
    ..CustomCampaigns/Utils/SpriteUtils.cs(132,13): warning CS0162: Unreachable code detected
    ..CustomCampaigns/Campaign/Missions/CompletionSubmission.cs(36,23): warning CS0618: 'UnityWebRequest.Post(string, string)' is obsolete: 'UnityWebRequest.Post with only a string data is obsolete. Use UnityWebRequest.Post with content type argument or UnityWebRequest.PostWwwForm instead (UnityUpgradable) -> [UnityEngine] UnityWebRequest.PostWwwForm(*)'
    ..CustomCampaigns/Managers/CustomCampaignUIManager.cs(87,50): warning CS0067: The event 'CustomCampaignUIManager.PropertyChanged' is never used
    ..CustomCampaigns/CustomMissionObjectives/CustomMissionObjectivesManager.cs(18,24): warning CS0169: The field 'CustomMissionObjectivesManager._config' is never used
    ..CustomCampaigns/UI/ViewControllers/ModalController.cs(21,50): warning CS0067: The event 'ModalController.PropertyChanged' is never used