Allow destroying budgets without investments

[bertocq]

Where

• Related Issue: Allow admins to delete Budgets #2275 (closes Allow admins to delete Budgets #2275)

What

Allowing admins to destroy budgets that have no associated investments from the Edit form.

How

• Adding a destroy button consistent with others in the admin sections
• Checking the Budget has no associated investments before destroy, showing an error notice explaining the reason behind if so.

Screenshots

When the Budget has no investments

When the Budget does have investments

Test

Extended existent admin budget feature spec adding a "Destroy" scenario that checks both a budget without investments and budget with investments destroyal.

Deployment

As usual

Warnings

None

[MariaCheca]

Nice gifs, by the way 🕶

[bertocq]

@MariaCheca all thanks to @justinfrankel 's LiceCap my kudos 👏