Make it possible to specify a default project template for a group
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### Problem to solve
It's hard to set default project settings for a group. You can create a project template, but project creators need to remember to use the correct template.
### Target audience
Whoever is responsible for setting group-wide project settings, and whoever creates projects in the group.
### Further details
- A team wants to use the same merge method for all projects in their group. Right now they can either remember to set the merge method when creating a new project, or create a template and then remember to use the template each time they create a project.
### Proposal
- Add a per-group setting for a default template, or "blank"
- Instead of always defaulting to a blank project, the new project flow will default to whatever was set as the default for that group.
### What does success look like, and how can we measure that?
Success looks like: one can create a new project in a group, specifying only the name (ie: *not* explicitly selecting a template or adjusting any other settings) and have certain settings set to non-default values based on the group (eg: the new project's merge strategy is set to "Merge commit with semi-linear history", if that's what the group wants)
### Links / references
@jeremy suggested that I file this in this reddit discussion: https://www.reddit.com/r/gitlab/comments/altfim/how_can_i_set_merge_method_for_all_future
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