Until now, uMap was not deleting files on delete, which can increase
file storage a lot after some time.
The files are not deleted, but moved to a "purgatory" folder, from
where they can be deleted after some time.
This allows to handle the loading of the settings in a consistant way,
and aditionnaly to provide a way to override the `WEBSOCKET_BACK_HOST`
and `WEBSOCKET_BACK_PORT` settings with arg commands `--host` and
`--port`.
Without this change, because of how we are currently loading our
settings, we would require the settings the be exposed by the
`umap.settings.__init__` file.
Previous implementations were exposing these settings, with the
following code:
```python
settings_as_dict = {k: v for k, v in globals().items() if k.isupper()}
```
It is now using `WEBSOCKET_BACK_HOST`, `WEBSOCKET_BACK_PORT` and
`WEBSOCKET_FRONT_URI`.
We need to take in consideration that the "front" WebSocket address
(that clients will connect to) might be different than the "back" ip and
port which are bound in the host.
This happens for instance for reverse proxies, or when running inside
a container.
We considered using a `WEBSOCKET_TLS` setting, to try guessing the
"front" address based on `WEBSOCKET_HOST`, `WEBSOCKET_PORT` and
`WEBSOCKET_TLS`, but as the back and front address can differ, this
would need to introduce a `WEBSOCKET_URI` in any case, so we went with
just using it, and not adding an extra `WEBSOCKET_TLS`.
For now we have only three modes:
- latest, which is the default and shows the last updated maps
- highlighted, which shows only the map that have been starred by a least
one staff member
- None, which does not show any map
