Allow default MR/issue template to be in repo

Currently, the default MR template is set from a projects general settings. Additional templates can be committed to the repo at .gitlab/merge_request_templates.

https://gitlab.com/help/user/project/description_templates#setting-a-default-template-for-issues-and-merge-requests-starter

When a user wants to update the default MR template, they may want to put the change in for review. You can do this with non-default templates, but not the default template. If the user doesn't have permission to edit the project, they wont even have access to the template and will have to open an issue to request its change.

I think we should allow for a default merge request template at /.gitlab/merge_request_templates/default.md.