Expiry and user quotas are controlled by four config options:
```expire_interval``` determines how often (in seconds) files should be
deleted. As every expiry run needs to check all uploaded files of all
users this should not be set too small.
Files older than ```expire_maxage``` (in seconds) will be deleted by an
expiry run. Set this to ```0``` to disable deletions based on file age.
After an expiry run at most ```user_quota_soft``` space (in bytes) will be
occupied per user. The oldest files will be deleted first until the occupied
space is less than ```user_quota_soft```. Set this to ```0``` to disable
file deletions based on a users occupied space.
```user_quota_hard``` sets a hard limit how much space (in bytes) a user
may occupy. If an upload would make his files exceed this size it will be
rejected. This setting is not dependend on ```expire_interval```. Set
this to ```0``` to disable upload rejection based on occupied space.
```expire_maxage``` and ```user_quota_soft``` depend on ```expire_interval```.
```user_quota_hard``` is honoured even without expiry runs. But some kind
of expiry is recommended otherwise a user will not be able to upload
anymore files once his hard quota is reached.
The difference between ```user_quota_hard``` and ```user_quota_soft```
determines how much a user may upload per ```expire_interval```.
Content-disposition: attachment prevents most code to be executed in the
browser (Java being an exception), but this is not needed for images and
makes viewing images in clients that don't show the pictures inline even
more complicated.
(Also this PR removes unused imports).
