Open-event-server: Add Foreign keys to Invoice model

[mrsaicharan1]

Is your feature request related to a problem? Please describe.
Currently, Event Invoice does not link to an order. This should be linked as it would help in the integration of invoice payments with the order workflow

Additional context
Working on it.

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[iamareebjamal]

Now that I think of it, this will require change in every query where Order is used and in relationship as well. This will be a no go. Please create a new table for it and add order as foreign key, which will both save the work and not break existing code

Reverting the PR

[mrsaicharan1]

@iamareebjamal Yes, I was dubious about this because of the order refactor.
Event Invoice table is already present so I'll just use it. I'll add the order foreign key.

[mrsaicharan1]

@iamareebjamal Just a question. The orders would still have to be recorded onto the Order table even with the foreign key, right?
Wouldn't it interfere in every query where Order is used(as this order is payment for event invoice)? Please correct me if I'm wrong.

[iamareebjamal]

Why will order be inserted? It'll already be there in the table, right?