Made Sources subscribeable per User

[benthebear]

• Make exitsting Sources subscribeable per User.
• Also make them unscribeable.

Added 18th Feburary:

• As I decided myself, the Categorization of Sources is bundled to the Subscription.
• That means, that the Subscription Table now has a column for the Cateogory.
• That also means, that you may choose from a category when your are subscribing to a given source.
• Also, within the Source-Add Dialog should be a Checkbox to directly subscribe to a new Source and give it a category.

[benthebear]

• Subscribeable they are.
• Categorizable they are.