Fix size threshold for "big email" codepath

The maximum size supported by QWebEngineView::setContent() is 2 MB, not 2 GB.

BUG: 387061
FIXED-IN: 5.15.0
1 job for !3 with release/20.08 in 11 minutes and 17 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Build
failed #59710
linux kf5-qt5 qt5.12

00:11:17

 
Name Stage Failure
failed
linux kf5-qt5 qt5.12 Build
make[2]: *** [messagecomposer/src/CMakeFiles/KF5MessageComposer.dir/build.make:824: messagecomposer/src/CMakeFiles/KF5MessageComposer.dir/composer/keyresolver.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [CMakeFiles/Makefile2:6923: messagecomposer/src/CMakeFiles/KF5MessageComposer.dir/all] Error 2
make: *** [Makefile:161: all] Error 2
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ERROR: Job failed: exit code 1