doc: add an issue template

This adds two new templates whenever an user decides to open a new issue:

Bug.md is a template for new bugs
Feature.md is a template for new features

Both files came from the gnome-todo project.

Fixes: #220

.gitlab/issue_templates/Bug.md

@@ -0,0 +1,20 @@
+Detailed description of the issue. Put as much information as you can, potentially
+with images showing the issue.
+
+Steps to reproduce:
+
+1. Open GNOME Calendar
+2. Create an event
+3. ...
+
+## Design Tasks
+
+* [ ]  design tasks
+
+## Development Tasks
+
+* [ ]  development tasks
+
+## QA Tasks
+
+* [ ]  qa (quality assurance) tasks

.gitlab/issue_templates/Feature.md

@@ -0,0 +1,17 @@
+Detailed description of the feature. Put as much information as you can.
+
+Proposed Mockups:
+
+(Add mockups of the proposed feature)
+
+## Design Tasks
+
+* [ ]  design tasks
+
+## Development Tasks
+
+* [ ]  development tasks
+
+## QA Tasks
+
+* [ ]  qa (quality assurance) tasks