Add user notification system

It would be good to have some kind of notification systems that automatically informs users of upcoming events, maintenance periods, upcoming changes to the platform, or anything else that is noteworthy.

I imagine some kind of banner at the top that can potentially be closed by users. The way that I imagine the implementation is that we add notifications to the registry that are specific to the domain, a time period, and potentially, some other user condition (e.g. checking for the existence for certain paths or so), and which the home app then automatically checks for at start up and presents to the user.

Here is a draft for how such a registry entry could look like:

```json
"notifications": [
{
"domain": "aiidalab-demo.materialscloud.org",
"after": "2020-12-24T00:00:00",
"before": "2020-12-25T00:00:00",
"message": "Merry Christmas!",
"category": "info",
}
]
```
Defaults would be:
```
"domain": ".*",
"after": ""
"before": ""
"category": "info",
```

Additional notes:

- There could be an optional "condition" object, that could be used for checks. I imagine that we implement certain checks that can be made, package version, existence of a specific file etc.
- The message should be accepted in (sanitized) HTML format to make it easy to customize notifications and embed images etc.