Update issue templates

In an attempt to enhance/professionalize our issue tracker, I've worked on some templates for issue submission. Please provide feedback.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Is something not working as it should? Please report it here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- READ THIS FIRST
+Thank you for reporting an issue with RawTherapee. Before you continue, please check the following:
+1. Have you searched through the open and closed issues on GitHub for keywords related to your issue? If somebody else has already reported your issue, do not report it again. This is particularly important if you found an issue in a release version of RawTherapee, because many bugs will have been fixed already in the latest development builds.
+2. Have you read http://rawpedia.rawtherapee.com/How_to_write_useful_bug_reports ? This page describes how you can obtain as much useful information as possible to help the developers to quickly fix your issue.
+3. If your problem is related to a specific image file or edit, always (!) provide the image and corresponding sidecar (pp3) file.
+-->
+
+**Short description**
+<!-- Write a brief summary of the issue. Remove this section if you can fit the summary in the title of the issue. -->
+
+**Steps to reproduce**
+<!-- Provide the necessary steps to reproduce the issue. Using a list is not necessary, but recommended. For example:
+1. Go to '...'
+2. Click on '....'
+3. Scroll down to '....'
+4. See error -->
+
+**Expected behavior**
+<!-- Explain what you observe, what you expected and why you think this is not correct behavior. -->
+
+**Additional information**
+<!-- Some essential information to provide along with your description:
+- Version of RawTherapee
+- Version of your operating system
+- If you built from source, the contents of the `AboutThisBuild.txt` file in the installation folder
+
+More information to provide:
+- Screenshots or screencast
+- Image file + sidecar (pp3)
+- Stacktrace from a debug build
+-->

.github/ISSUE_TEMPLATE/camera-support---color-calibration.md

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+---
+name: Camera support / color calibration
+about: Report new file formats or requests for color calibration support here
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- If files from your camera cannot be opened by RawTherapee, or the colors do not look right, you can provide us with information to improve support for your camera. Before requesting making a request, please read http://rawpedia.rawtherapee.com/Adding_Support_for_New_Raw_Formats and make sure to provide the necessary shots.
+If you own a color calibration target (such as an X-Rite ColorChecker Passport), you can further improve the color rendition of files from your camera. Please read http://rawpedia.rawtherapee.com/How_to_create_DCP_color_profiles and provide the necessary shots.
+
+Requests for support or color calibration without providing the necessary information will be deleted. -->

.github/ISSUE_TEMPLATE/camera-support.md

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+---
+name: Camera support
+about: Report new file formats or requests for color calibration support here
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!--

.github/ISSUE_TEMPLATE/feature-request---enhancements.md

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+---
+name: Feature request / enhancements
+about: Ideas and suggestions for improvement are always welcome!
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!-- READ THIS FIRST
+Please be advised that our current capacity for development is limited. Suggestions are always welcome, but we cannot guarantee that they will be looked into or implemented.
+Also make sure that you have searched through the open and closed issues on GitHub for keywords related to your suggestion. If somebody else has made the same suggestion, please do not report it again.
+-->
+
+<!-- Give a clear and complete description of WHAT your suggestion improvement is, WHY you think this would be useful or better, and, if possible, give as much input to describe HOW the improvement should be implemented. -->