Add GitLab issue templates for bugs and features.

.gitlab/issue_templates/New feature.md

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+** Feature summary **
+
+(Describe what you would like to be able to do with Geary that you
+currently cannot do, such as "send email whilst underwater". Please
+fill in all text in parentheses like this.)
+
+** How would you like it to work? **
+
+(If you can think of a way Geary might be able to do this, let us know
+here.)
+
+** Relevant links, screenshots, etc **
+
+(If you have further information about the missing feature such as
+technical documentation or a similar feature in another email
+application, please provide links and/or screenshots.)
+
+/label ~Feature