Add the option to NOT push after commit on Submit when using LFS

[sinbad]

Over time I've found it annoying that when using LFS locking, Submit to Source Control always pushes. The reasons I might not want to:

• I have a sequence of commits to make and push each time slows that down
• I know the push will fail because I'm working disconnected or behind the remote branch
• I want to combine asset changes with C++ changes in one commit; without push I can amend the UE commit & add the C++ changes before push

The only reason for always pushing seems to be to unlock files, but I already implemented unlock-on-push in #137, so this is not a hard limit any more. So long as you do the final push in UE the files from the intermediate commits will be unlocked.

So I've added an extra option in the source control setup dialog to disable auto-pushing. It defaults to the previous behaviour.

[SRombauts]

Hey @sinbad, thanks a lot for all your awesome changes!
I wasn't available for quite a long time, with a baby at home, covid/wfh etc but I am now trying to come back to open source dev :)

[sinbad]

Welcome back :)