Use pessimistic locking in budget

[josepjaume]

🎩 User Story

We've found out that budgfets are vulnerable to going over the limits by clicking in multiple projects sequentially, very fast.

This should be tackled in two ways:

• Add pessimistic locking against Order when adding projects in the command. Documentation: http://api.rubyonrails.org/classes/ActiveRecord/Locking/Pessimistic.html
• Add a validation on Order that ensures the amount of projects doesn't exceed the total amount.

📋 Related documentation

None

🎯 Acceptance criteria

None

📌 Related issues

None

[andreslucena]

Duplicated #862 (although this one is better explained, so I'm closing the other one)

[josepjaume]

Oh, sorry! Also, the two ways are compatible and should both be implemented.

[andreslucena]

Also addno this validations on the javascript side.

[josepjaume]

That's not easy @andreslucena as we're storing the state on the DOM (using ujs). That would be manegeable using react but the app's achitecture can't currently hold it. Anyway, this will be transparent for the user as of #881