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

@@ -0,0 +1,32 @@
+---
+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).