Adds a checkbox for mount option override, fixes #520

[ghost]

There are 3 issues with this pull:

1. Adding the checkbox adds clutter to the UI
2. What should the default setting should be, as either setting can break permissions, see Override permissions for borg mount #362 and Use different permissions when mounting archive. #520.
3. Does the average user knows what mount permissions are? Possible solution would be add a tooltip along the likes of "Toggle this if any permission denied errors pop up".
   I'd like some input before going further, mostly on issue 2

[ghost]

After thinking about it, I think the default behaviour should be more like unmodified borg mount command, so I'll make the checkbox unticked by default

[ghost]

Not sure if its state should be saved or not.

[samuel-w]

Also closes #436

[samuel-w]

Replaced by #682