Add "Project proposal" issue template

See #8

.gitlab/issue_templates/Proposal.md

+<!-- Read our mentor's guidelines at https://wiki.gnome.org/Outreach/SummerOfCode/Mentors -->
+
+# Project name:
+
+# Mentors:
+<!-- preferably gitlab/ldap usernames -->
+
+# Description:
+<!-- include the list of components (apps, libraries) that need to be touched in this project -->
+
+# Requirements:
+<!-- pre-requisites such as: knowledge of a specific programming language or technology -->
+
+# Communication:
+<!-- preferred communication channel: chat (IRC/Matrix), email, video call, etc... -->
+
+/label ~"Project proposal"