Update create_desktop_file.sh with categories

[dkadioglu]

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Summary
Add "Categories=Network;InstantMessaging;" to ensure that Mattermost is listed under the appropriate submenu of the application starter.

Test Cases
Tested with KDE/Plasma 5.46/5.12.5 and KMenu. With the change, Mattermost is listed under "Internet", without the change Mattermost is listed under "Lost & Found".

[mattermod]

Thanks @dkadioglu for the pull request!

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[jasonblais]

Thanks for signing the CLA! Requesting first review from @yuya-oc

[yuya-oc]

Looks good to me on Ubuntu 16.04+GNOME Classic. Mattermost moved from "Other" to "Internet".