Add GitLab issue templates for bugs and features.

.gitlab/issue_templates/New bug.md

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+** Bug summary **
+
+(Summarise the bug encountered concisely here. Please fill in all text
+in parentheses like this.)
+
+** About you **
+
+ * Geary version: (x.y.z)
+ * Installation method: (Package? Flathub? Source code?)
+ * Your desktop: (GNOME? KDE? Something else?)
+ * Your operating: system and version (Ubuntu 16.04? Fedora 28? Rolled your own?)
+ * Email provider: (Gmail, Yahoo!, Outlook.com, or someone else?)
+
+** Steps to reproduce**
+
+ 1. (Step one)
+ 2. (Step two)
+ 3. (...)
+
+** What is the current bug behavior? **
+
+(What actually happens)
+
+** What is the expected correct behavior? **
+
+(What you see happening instead)
+
+** Relevant logs and/or screenshots **
+
+(Paste any relevant logs - please use code blocks (```) to format
+console output, logs, and code as it's very hard to read otherwise. If
+the bug is a crash, please obtain a [stack
+trace](https://wiki.gnome.org/Apps/Geary/ReportingABug#Stack_Trace)
+and attach it to this bug.)
+
+/label ~Bug

.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