NOISSUE Use constants for user agents

Here lies yet another early-stage move to debrand the MultiMC codebase,
as well as reducing the burden of updating strings across the codebase
for a future MultiMC6.

api/logic/screenshots/ImgurAlbumCreation.cpp

@@ -20,7 +20,7 @@ void ImgurAlbumCreation::start()
 {
     m_status = Job_InProgress;
     QNetworkRequest request(m_url);
-    request.setHeader(QNetworkRequest::UserAgentHeader, "MultiMC/5.0 (Uncached)");
+    request.setHeader(QNetworkRequest::UserAgentHeader, BuildConfig.USER_AGENT_UNCACHED);
     request.setHeader(QNetworkRequest::ContentTypeHeader, "application/x-www-form-urlencoded");
     request.setRawHeader("Authorization", "Client-ID 5b97b0713fba4a3");
     request.setRawHeader("Accept", "application/json");