Implement roles management (#18)

Fix formatting

Implement `roles` management

Add a 'roles' mod and DB

youmubot-core/src/db.rs

@@ -8,6 +8,9 @@ use youmubot_db::{GuildMap, DB};
 /// A list of SoftBans for all servers.
 pub type SoftBans = DB<GuildMap<ServerSoftBans>>;
 
+/// A list of assignable roles for all servers.
+pub type Roles = DB<GuildMap<HashMap<RoleId, Role>>>;
+
 /// For the admin commands:
 ///  - Each server might have a `soft ban` role implemented.
 ///  - We allow periodical `soft ban` applications.
@@ -34,3 +37,10 @@ pub struct ImplementedSoftBans {
     /// List of all to-unban people.
     pub periodical_bans: HashMap<UserId, DateTime<Utc>>,
 }
+
+/// Role represents an assignable role.
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct Role {
+    pub id: RoleId,
+    pub description: String,
+}