Add "submit button label" option to EntityForm

[Martinsos]

We want to be able to specify button label for EntityForm submit button.

Probably most relevant piece of code to start from is here: https://github.com/wasp-lang/wasp/blob/master/waspc/src/Wasp/EntityForm.hs#L54 -> here it should be added as Submit button option.
There are really three parts to this change:

1. As mentioned above, expand Wasp.EntityForm with submit button label option.
2. Modify parser so it parses this option.
3. Modify generator so it appropriately renders this button.

We should also remove, from generated code, option to modify this label, which is currently there -> it is not needed anymore if we do this.

While this issue requires changes to the whole system, I believe it is good "starter" issue as it can be mostly done by copying how it is done for other EntityForm options, is conceptually simple, all is clear regarding requirements, and it teaches you about the whole waspc.

[Martinsos]

Not relevant any more!