feature: concurrent deploy

[vzhemchuzhnikov]

Currently, if two users edit scripts, flows or apps at the same time, the last who is saving will overwrite any change made by the first user.
Multiplayer feature is somewhat helps, but:

1. It only works with scripts
2. It only available in EE

Even with the multiplayer feature enabled, it is possible to have a race condition.

At the bare minimum, whenever we are saving a file in Windmill, we could check if the currently deployed file hash is the one we started with. If it is not, we could reject a save and ask to manually merge changes to the latest version (e.g. with wmill cli).
But it would be even better if we could do this merging in the UI as well (this could also be an EE feature).