Allow saving issues as drafts
### Description
As a project manager I'd like to be able to save an issue that is not visible for others until I officially publish it.
That allows me to start drafting an issue and finish it when all requirements are known.
### Proposal
* Add a submit-button `store issue as draft` (besides `Submit issue` and `cancel`)
* Add a tab `Drafts` to the issue list (besides `Open`, `Closed` and `All`)
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