Include icon for image data

[k-nut]

Fixes #2948, #2910

Proposed fixes:

In order to clean up Pull Requests a little I just took the changes from #2948 and applied the to the lastest master. Of course this still does not include the original vector files as I do no have them.

Features:

• includes tests covering changes
• includes updated documentation
• includes user-visible changes
• includes API changes
• includes bugfix for possible backport

[k-nut]

In #2910 @amercader pointed out that the original files seem to be in this repository: https://github.com/mintcanary/file-types
That repository does not include the new image-type icon though. Maybe @hknochi can tell us where he got it from or provide the original?

[k-nut]

@amercader I think we should merge this rather soon instead of waiting for the original vector files. Waiting for them led to the last PR becoming conflicting with the current master. I could create a separate issue for also including the vector files?

[hknochi]

@k-nut : i think to add the vector files in the repo won't be the best idea (may these should be in a other/single one)

[k-nut]

Yes. We might want to store them in a separate repo. But at least having something like a link to the original somewhere in the ckan project could prove useful already.

[hknochi]

okay i'll add the source . you can move them to a custom path afterwards

[hknochi]

please take a look here: https://drive.eccenca.com/d/73541a96f2/

[k-nut]

@hknochi cool, thank you! How are the icons licensed?

[k-nut]

@amercader do you think we should create a new repository for design assets like this? Or is there something already where we could put this? Or would you just include it in this repository?

[seebi]

Fromm eccenca's POV this should not be decided by us but by @smth who originally pushed these files (mintcanary/file-types@1740cd6). The work of @hknochi can be licensed under any terms suitable for you.

[smth]

I just added a license here.

[k-nut]

Created a first pull request in the new ckan-assets repository. Once that is merged this PR should also be good to merge.