Consolidate license endpoints + DTOs

Currently we have a collection of different endpoints and DTOs serving similar purposes:

* `GET /billing` (used in `AdminSettings.vue`):
```json
{
"hubId": "hub3000",
"email":, "admin@example.com",
"licensedSeats": 42,
"usedSeats": 23,
"issuedAt": "2020-01-01T00:00:00Z",
"expiresAt": "2030-01-01T00:00:00Z",
"managedInstance": false
}
```
* `GET /license/user-info` (used in `VaultList.vue` and `VaultDetails.vue`):
```json
{
"licensedSeats": 42,
"usedSeats": 23,
"expiresAt": "2030-01-01T00:00:00Z",
}
```
* `GET /config/` (used in whole app):
```json
{
"keycloakUrl": "https://example.com/kc",
"...":, "more stuff",
"entitlements": {
"seats": 42,
"...": "more entitlements"
}
}
```