Store all files only once

[Mikaela]

Pre-#232 this was done by disabling randomization which now results to #237.

In #232 @Bysmyyr commented:

sounds good, the better way should be hash the file to get id for it->same file stored only once.

[Mikaela]

(Labeling also as a bug, because the same file shouldn't be uploadable many times as a different file to take space.)

[Tuupertunut]

The latest development version now has a hash-based link cache for imgur links. (see imgur-utils.js) That could be used as a reference implementation.