Extract GIN changes from templates into importable sub-templates

An example of this that we did recently is the repository header (see PRs #78 and #81), where we created [header_gin.tmpl](https://github.com/G-Node/gogs/blob/master/templates/repo/header_gin.tmpl) and import it into [header.tmpl](https://github.com/G-Node/gogs/blob/85a02127d3b7dc06bf301ec08c6bf1a3fc8f6e86/templates/repo/header.tmpl#L23)

The template at [templates/repo/view_file.tmpl](https://github.com/G-Node/gogs/blob/master/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.