Add software license to software installation

[eric-nieuwland]

Please add the ability to register the software license used for a software installation. Preferably, allow for the software license to be shared among installations.

License would include:

• kind/type (e.g. GPL, CC, single payment, repeated payment)
• expiration date that can be queried (including NULL iff not paid license)

NOTE Actual license keys should not be stored here. They should be kept in a separate secure place. A reference to that place could be included here.

[MartVisser]

Suggested fields and links to other models:

• Software Version (Link to Software Version model)
• Type
• Start Date (Optional)
• Expiration Date (Optional)
• Installation (Link to Software Installation model)
• Stored Location (Where is the license key stored? - Maybe create a separate model for this)