feat(auth): Add role-based access management from OpenID Connect (#535)

* feat(auth): Add role-based access management from OpenID Connect

Signed-off-by: Marvin A. Ruder <signed@mruder.dev>

* Apply suggestions from code review

Signed-off-by: Marvin A. Ruder <signed@mruder.dev>

---------

Signed-off-by: Marvin A. Ruder <signed@mruder.dev>

backend/src/oauth/provider/discord.provider.ts

@@ -101,10 +101,10 @@ export class DiscordProvider implements OAuthProvider<DiscordToken> {
       });
       const guilds = (await res.json()) as DiscordPartialGuild[];
       if (!guilds.some((guild) => guild.id === guildId)) {
-        throw new ErrorPageException("discord_guild_permission_denied");
+        throw new ErrorPageException("user_not_allowed");
       }
     } catch {
-      throw new ErrorPageException("discord_guild_permission_denied");
+      throw new ErrorPageException("user_not_allowed");
     }
   }
 }