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)
Status Job ID Name Coverage
failed #59710
linux kf5-qt5 qt5.12


Name Stage Failure
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
Uploading artifacts for failed job
Uploading artifacts...
WARNING: JUnitTestResults.xml: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1