Add crop icons to Crop, for use elsewhere in the program

[Tropicalrambler]

Problem statement

Users can select any crop they seem fit. However, to make the application reports more "visually appealing in the long term", allowing the user to select (and eventually contribute to) from a pre-existing set of crop icons would make map or 2d reports intuitive and provide a lot of information at a glance. The table reports could also have icons, for a quick visual reference the farmer can use.

Challenges

Eventually, the icons could have several states (with relevant timeframes to their species) "Recently Planted" "1 week old" "2 weeks old" "Ready to harvest" "Just pruned" "After harvest" "Harvested"

Possible solution to Problem

Although not many icons have been uploaded, we could probably fork from FarmBot's set of icons, and then merge to openfarm. Or, given CC0, merge into agriculture in ERPNext. https://github.com/openfarmcc/open-crop-icons

Here are the explanations and the status of more icons by @roryaronson https://github.com/openfarmcc/open-crop-icons/issues/2

And design Guidelines: https://github.com/openfarmcc/open-crop-icons/issues/8

And as all open source projects are always a work in progress, they have NOT uploaded all of them to their repo, but they have a decent list here: https://drive.google.com/drive/folders/0Bzm0yyP7JMLiM0U4ZkdmNG9rVjQ?usp=sharing

Reference screenshots / mockups (if any)

Checklist :

• [ ] Finalized on solution (out of several)
• [ ] Finalized on solution implementation (of one)
• [ ] Implemented solution (as code)
• [ ] Made PR (link to PR here please)