Ataki/gridfs

@stvstnfrd to review commit
The way this is structured, we're performing the `String.format`
_before_ translating the string. What we want is to translate the string
and _then_ perform the string replacement. So:
```python
{
    'error': _('File exceeds max size of {filesize_in_mb} MB.').format(
        filesize_in_mb=10,
    )
}
```