Add Wallets

[emarj]

Every wallet will have a capital/amount inside (#23)

[emarj]

Wallets should be shareable, i.e. they could be owned by multiple users

type Wallet struct {
UUID uuid.UUID
Name string
Description string
Owners []*User
Amount decimal.Decimal
Private bool
Methods []*Method
}

This would require 3 tables: wallets,wallets_users,wallets_methods.

(Also need wallets_categories?)