Merge branch 'add_templates' into 'master'

chore: add MR and issue templates

See merge request pubsweet/pubsweet!515

.gitlab/issue_templates/Bug.md

+**Describe the bug**
+
+...
+
+**Steps To Reproduce**
+
+**Expected behavior**
+
+...
+
+**Screenshots**
+
+If applicable, add screenshots to help explain your problem.
+
+**Device, Operating System and Browser**
+

.gitlab/merge_request_templates/Default.md

+#### Background
+
+What does this MR do?
+
+#### Any relevant issue
+
+Please use the phrase 'closes' if it will close an issue.
+
+#### How has this been tested?
+
+**Reviewers** ensure that these test requirements are also reviewed.
+
+Please indicate the need for this change to be tested. If not please justify, if so then at what levels. 
+
+- [ ] Unit / Integration tests
+- [ ] Browser tests
+- [ ] E2E tests
+
+#### Documentation updates
+
+- [ ] New features to document
+- [ ] Updating existing documentation if needed
+