Accounts: Unlink Telegram/Discord/Matrix through cog

Added an unlinking section to the little cog dropdown next to users so
that one can remove and re-link a different account for a Jellyfin user.
Also adjusted padding in the dropdown.

models.go

@@ -355,3 +355,7 @@ type setAccountsAdminDTO map[string]bool
 type genCaptchaDTO struct {
 	ID string `json:"id"`
 }
+
+type forUserDTO struct {
+	ID string `json:"id"` // Jellyfin ID
+}