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 release/20.08 in 32 minutes and 50 seconds
Status Job ID Name Coverage
failed #59717


Name Stage Failure
build_clazy_clang_tidy Build
1 error generated.
make[2]: *** [messagecomposer/src/CMakeFiles/KF5MessageComposer.dir/build.make:811: messagecomposer/src/CMakeFiles/KF5MessageComposer.dir/composer/composerviewbase.cpp.o] Error 1
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