No way to delete unique constraint when it was created with a custom name

[Mordil]

When creating a .unique constraint using the unique(on:name:) method, the constraint name will use the given name value, rather than an automatically generated one.

When attempting to delete the unique constraint in revert(on:) using the deleteUnique(on:) method, it will attempt to delete the constraint with an automatically generated name from the unique keys, rather than the custom name provided in the unique(on:name:) method - which causes the migration revert to fail.

To Reproduce

1. Create a new migration
2. Add a .unique constraint using .unique(on:name:)
3. In the revert(on:) method, call .deleteUnique(on:)
4. Run swift run migrate -y
5. Run swift run migrate --revert -y

You will see the following error:

$ vapor run migrate --revert -y
Migrate Command: Revert
The following migration(s) will be reverted:
- App.Tool.Seed on default
- App.Tool.MakeToolUnique on default
- App.Tool.AddMaker on default
- App.Tool.Create on default
Would you like to continue?
y/n> yes
[ WARNING ] previousError(server: constraint "uq:tools.name+tools.maker" of relation "tools" does not exist (ATExecDropConstraint))
Swift/ErrorType.swift:200: Fatal error: Error raised at top level: previousError(server: constraint "uq:tools.name+tools.maker" of relation "tools" does not exist (ATExecDropConstraint))