Use a generic markup class to display externally rendered files and diffs (#15735)

* creates and implements generic markup less class

* How to give custom CSS to externally rendered html

* Clarifies sources of CSS styling of markup

* further clarification of sources of markup styling

* rename _markdown to _markup

* remove defunct import

* fix orphaned reference

* Update docs/content/doc/advanced/external-renderers.en-us.md

* more renames markdown -> markup

* do not suggest less customization

* add back tokens

* fix class whitespace, remove useless if-clause

* remove unused csv-data rules

* use named exports and rename functions

* sort imports

Co-authored-by: HarvsG <11440490+HarvsG@users.noreply.github.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: silverwind <me@silverwind.io>

templates/repo/empty.tmpl

@@ -27,7 +27,7 @@
 
 							<div class="item">
 								<h3>{{.i18n.Tr "repo.create_new_repo_command"}}</h3>
-								<div class="markdown">
+								<div class="markup">
 									<pre><code>touch README.md
 git init
 {{if ne .Repository.DefaultBranch "master"}}git checkout -b {{.Repository.DefaultBranch}}{{end}}
@@ -41,7 +41,7 @@ git push -u origin {{.Repository.DefaultBranch}}</code></pre>
 
 							<div class="item">
 								<h3>{{.i18n.Tr "repo.push_exist_repo"}}</h3>
-								<div class="markdown">
+								<div class="markup">
 									<pre><code>git remote add origin <span class="clone-url">{{if $.DisableSSH}}{{$.CloneLink.HTTPS}}{{else}}{{$.CloneLink.SSH}}{{end}}</span>
 git push -u origin {{.Repository.DefaultBranch}}</code></pre>
 								</div>