Templates for extra links in top navbar and repo tool tabs. (#3308)

These templates add content to the top navbar
and to the repository page.
This way we do not have to copy and modify the whole
template, and re-modify it after upgrading Gitea if
it changes.

Signed-off-by: Alberto González Palomo <bugs@sentido-labs.com>

docs/content/doc/advanced/customizing-gitea.en-us.md

@@ -66,6 +66,8 @@ To add custom HTML to the header or the footer of the page, in the `templates/cu
 directory there is `header.tmpl` and `footer.tmpl` that can be modified. This can be
 a useful place to add custom CSS files or additional Javascript.
 
+If all you want is to add extra links to the top navigation bar, or extra tabs to the repository view, you can put them in `extra_links.tmpl` and `extra_tabs.tmpl` inside your `custom/templates/custom/` directory.
+
 ## Customizing gitignores, labels, licenses, locales, and readmes.
 
 Place custom files in corresponding sub-folder under `custom/options`.

templates/base/head.tmpl

@@ -158,6 +158,9 @@
 					{{end}}
 
 					<a class="item{{if .PageIsExplore}} active{{end}}" href="{{AppSubUrl}}/explore/repos">{{.i18n.Tr "explore"}}</a>
+
+					{{template "custom/extra_links" .}}
+
 					{{/*
 						<div class="item">
 						<div class="ui icon input">

templates/repo/header.tmpl

@@ -89,6 +89,8 @@
 				</a>
 			{{end}}
 
+			{{template "custom/extra_tabs" .}}
+
 			{{if .IsRepositoryAdmin}}
 				<div class="right menu">
 					<a class="{{if .PageIsSettings}}active{{end}} item" href="{{.RepoLink}}/settings">