SPDX integration

[DorsalGreen]

As you suggested, I kept the dejacode licenses and the SPDX license keys separate.
I modified the template to honor both license keys when checking if the license text should be put at the end of the document.

[chinyeungli]

Thanks for the commit. However, I am not in a favor to have 2 separate lists of Common Licenses.
In addition, these 2 lists are not in sync which may cause problems. (i.e. one states it's under a common license while the other doesn't) Furthernmore, it'll take extra steps for maintenance such that if some licenses need to be removed/added/updated in the common license list.
We have a plan/idea to ask scancode-toolkit to provide an API to get the SPDX license based on the scancode's license_key. That way, we don't need to worry about (or maintain) the SPDX license here.

[OctagonHex]

Please close or delete this pull-request. I don't have the rights to do so.