Create 'Feature' Issue type

Add a 'Feature' issue type that is used for 'bigger' tasks. Only Approvers should be allowed to create features.

Reasoning:
We're missing a general way to collect & manage new features & important changes. The idea is that looking at all the 'Feature' issues for a certain component should give you a good high-level overview about extensions & important changes planned for a component. Feature tasks would usually be managed by the component or submodule maintainer, would be discussed on an appropriate mailing list, and would be used to collect feedback, reasoning for the feature, final decisions etc.

Comparison to other issue types:

Bug - Filed also by end-users, contains usually smaller issues
Task - Filed also by end-users, very generic scope, is often used as personal 'TODO' list
Research - Only for research items, very generic scope. Might end up in a 'Task', or 'Feature' when actually being implemented.
Suggestion - Filed also by end-users, very generic scope. Might end up in aa 'Task', or 'Feature' when accepted & actual implementation starts.