Replace underscores in dates with spaces before parsing

This allows for parsing date **and time** from the filename
```
FILENAME_METADATA = '(?P<date>\d{4}-\d{2}-\d{2}(_\d{2}:\d{2})?)_(?P<slug>.*)'
```
The format version with `_` is somewhat nicer and more readable than the one with a `T`.
It's an edgy use case but the drawbacks are few, if any.