Add button to output page to download output as a file

[jonblack]

The user should be able to download the output as a file. If the output is an error message, the download button should be disabled.

[jonblack]

There are two ways to achieve this:

1. POST the result in the output back to the server so it can send it back as a file;
2. Save the result output to a file and link to a download URL which serves the saved file;

The first option is a bit ugly. The second option requires a means to manage the storage of results - after a certain amount of time, the data should be deleted (e.g. one week).

Unless there's a good reason not to, implement number 2. Postpone for now.

[jonblack]

Duplicate of #21.