[gitlab] Add templates for issues and feature requests / merge requests

.gitlab/issue_templates/Bug.md

+<!--
+This form is for bug reports ONLY!  
+If you're looking for help check out the [readme](/README.md).
+-->
+**Bug report**
+
+**What happened / Problem description**:
+
+**What you expected to happen**:
+
+**How to reproduce it (as minimally and precisely as possible)**:
+
+**Anything else we need to know?**:
+
+**Environment**:
+- Dune version:
+- DuMux version:  
+- Others:

.gitlab/issue_templates/Feature.md

+<!--
+This form is for feature requests ONLY!
+If you're looking for help check out the [readme](/README.md).
+-->
+**Feature request**
+
+**What does this feature / why does DuMux need it**:
+
+**Which issue does this feature fix (if any)**
+
+**Anything else we need to know?**:

.gitlab/merge_request_templates/Merge Request.md

+<!--
+Thanks for sending a merge request!  
+If this is your first time, read our [contributing guidelines](/CONTRIBUTING.md)
+-->
+**What this MR does / why does DuMux need it**:
+
+<!--
+**Which issue this MR fixes** *(optional - uncomment and add issue)*:
+fixes #
+-->
+
+**Special notes for your reviewer**: