API rework (v2)

[lus]

This PR will create a whole new API version (API v2) with new features and concepts implemented.
API v1 won't be removed just yet but may in the future.

API v2 will implement the backend basis for the following new features:

• The so far named deletion token will be renamed to modification token to allow more extendability (basis for Edit pastes using deletion token #10)
• Pastes will be editable (Edit pastes using deletion token #10)
• Pastes will have a key-value store for meta data, for example for frontend-specific stuff (basis for Paste encryption #11)
• A configuration option for a webhook being called whenever a paste is reported will be added (Report inappropriate pastes #12)
• Furthermore, all API endpoints and behaviours will be documented (Api for Dev s #14).

Frontend functionality will still be based on API v1 and thus will be restricted to the old functionalities after merging.
A frontend update will follow soon after that.