UI breaks if the value of "serviceEndpoint" is an object

The frontend UI incorrectly assumes that the value of serviceEndpoint is always a string. It should also be able to handle cases where the value is an object (see [Service Endpoints in DID Core](https://w3c.github.io/did-core/#service-endpoints)).

E.g. if a DID document contains this:

```
"service": [{
"id": "PersonalInfo",
"type": "AgentService",
"serviceEndpoint": {
"type": "UserServiceEndpoint"
}
}],
```

then React gives the following error:

```
Objects are not valid as a React child (found: object with keys {type}). If you meant to render a collection of children, use an array instead.
```