DON'T MERGE: Delete unused files

[elsbrock]

These files are included in our repository but referenced nowhere.

Please don't merge this pull request, it is created only for previewing the change on http://pr-116.rzl-homepage.raumzeitlabor.org.

It is probably a good idea to run the unused-images script again.

closes #115

[abrock]

The linkchecker doesn't find any missing images in the preview except for /assets/power_cell.png in http://pr-116.rzl-homepage.raumzeitlabor.org/blog/projekt-solar-powered-router/ which was already missing before, so I think it should be safe to force-push the change.

[elsbrock]

Thanks, will do later.

[elsbrock]

So, I just tried this using bfg. Here's what I did:

1. git clone --bare
2. du -sh: 262 MB
3. run bfg
4. expire reflog and trigger git gc
5. du -sh: 259 MB

Unsure why we only save 3 MB. Also, bfg only supports deleting by filename (without path). Some of the files in app/assets/upload are located under app/assets and are actually used, so we should be careful here.

[abrock]

I managed to shrink it to 175MB: https://github.com/abrock/rzl-homepage

I took my local (slightly outdated) version where the latest commit is "Add missing image files...", then compressed all the png files using optipng -o7 and deleted every orphaned file.
Then I ran bfg etc.