Add API call for frontend to fetch list of valid roles from backend

- only relevant for admin config (addUser, editRole)
- admin only (double-checked)

web/frontend/src/config/admin/EditRole.svelte

@@ -8,6 +8,15 @@
     let message = {msg: '', color: '#d63384'}
     let displayMessage = false
 
+    export let roles = []
+    let roleLabel = {
+      api: 'API',
+      user: 'User',
+      manager: 'Manager',
+      support: 'Support',
+      admin: 'Admin'
+    }
+
     async function handleAddRole() {
         const username = document.querySelector('#role-username').value
         const role = document.querySelector('#role-select').value
@@ -86,11 +95,9 @@
             <input type="text" class="form-control" placeholder="username" id="role-username"/>
             <select class="form-select" id="role-select">
                 <option selected value="">Role...</option>
-                <option value="user">User</option>
-                <option value="manager">Manager</option>
-                <option value="support">Support</option>
-                <option value="admin">Admin</option>
-                <option value="api">API</option>
+                {#each roles as role}
+                    <option value={role}>{roleLabel[role]}</option>
+                {/each}
             </select>
             <!-- PreventDefault on Sveltestrap-Button more complex to achieve than just use good ol' html button -->
             <!-- see: https://stackoverflow.com/questions/69630422/svelte-how-to-use-event-modifiers-in-my-own-components -->