Adds issue creation templates for bugs, features, proposals and documentation.

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* Use this issue template for reporting a new bug.

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### Summary

### Steps to reproduce:

1. step 1

2. step 2

3. ...

### What is the current bug behavior?

### What is the expected behavior?

### Environment

<!--

Please fill in the following information in bug reports, removing the comments like this one in brackets.

-->

- Version: e.g. 0.10, output of --version (without backslashes)

- Operating system: Debian GNU/Linux 9.1 (stretch), Windows 7, Ubuntu Xenial, etc

- Install method: distribution package, from source tarball, from git, etc

- etc...

### Relevant logs and/or screenshots:

### Possible fixes:

/label ~bug

<!--

* Use this issue template for suggesting new docs or updates to existing docs.

-->

### Problem to solve

<!-- Include the following detail as necessary:

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* What feature(s) affected?

* What docs or doc section affected? Include links or paths.

* Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs?

* Any other ideas or requests?

### Further details

<!--

* Include use cases, benefits, and/or goals for this work.

* If adding content: What audience is it intended for? (What roles and scenarios?)

-->

### Proposal

<!-- Further specifics for how can we solve the problem. -->

### Who can address the issue

<!-- What if any special expertise is required to resolve this issue? -->

### Other links/references

<!-- E.g. related Tor issues/MRs -->

/label ~documentation

<!--

* Use this issue template for submitting a feature.

-->

### Summary

### What is the expected behavior?

/label ~Feature

<!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.-->

### Proposal

<!-- Use this section to explain the proposal and how it will work. It can be helpful to add technical details, design proposals, and links to related epics or issues. -->