Extract GIN changes from templates into importable sub-templates

[achilleas-k]

An example of this that we did recently is the repository header (see PRs #78 and #81), where we created header_gin.tmpl and import it into header.tmpl

The template at templates/repo/view_file.tmpl in particular should be cleaned up.

Related to #60, but a different strategy. This approach might also be a good way to start towards implementing #60 at a later date.