Update issue templates

Define new issue templates following new structure for them.

Also add a new template related to site config.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+### Issue details
+
+Please provide issue details here.
+Remember, this is _not_ a place to ask questions. For that, go to our chat at http://gitter.im/wallabag/wallabag or https://matrix.to/#/#wallabag_wallabag:gitter.im.
+
+### Environment
+
+* wallabag version (or git revision) that exhibits the issue:
+* How did you install wallabag? Via `git clone` or by downloading the package?
+* Last wallabag version that did not exhibit the issue (if applicable):
+* php version:
+* OS:
+* type of hosting (shared or dedicated):
+* which storage system you choose at install (SQLite, MySQL/MariaDB or PostgreSQL):
+* <details>
+  <summary>Paste your `app/config/parameters.yml` below</summary>
+  PASTE HERE
+</details>
+
+### Steps to reproduce/test case
+
+Please provide necessary steps for reproduction of this issue, or better the
+reduced test case (without any external dependencies, if possible).

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: Feature
+assignees: ''
+
+---
+
+**Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
+
+**Describe the solution you'd like**
+A clear and concise description of what you want to happen.
+
+**Describe alternatives you've considered**
+A clear and concise description of any alternative solutions or features you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/ISSUE_TEMPLATE/fetching-content.md

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+---
+name: Fetching content
+about: If wallabag can't extract content for an URL
+title: Wrong display in wallabag (HOST)
+labels: Site Config
+assignees: ''
+
+---
+
+**Before submitting the issue, please read:**
+If wallabag can't parse / extract content for a given link, please first read the documentation about it:
+http://doc.wallabag.org/en/user/errors_during_fetching.html#how-can-i-help-to-fix-that
+
+We have a lot of request about fetching config issue. It'll help us a lot if you give a try to fix it on your own following the doc.
+If you failed to fix it yourself, tick the following box:
+- [ ] I've tried myself without success
+- [ ] I've replaced `HOST` in the issue title with the host of the URL that can't be fetched (ie: `nytimes.com`, `20minutes.fr`, `bbc.com`, etc.)
+
+**Content related:**
+ - URL: [full url of the content]
+ - wallabag version: [e.g. 2.4.0]
+
+**Describe what's wrong:**
+A clear and concise description of what you expected to happen.

.github/ISSUE_TEMPLATE/fetching-issue.md

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+---
+name: Fetching issue
+about: If wallabag can't extract content for a content
+title: ''
+labels: Site Config
+assignees: ''
+
+---
+
+If wallabag can't parse / extract content for a given link, please first read the documentation about it:
+http://doc.wallabag.org/en/user/errors_during_fetching.html#how-can-i-help-to-fix-that