Add download button for file viewer (#17640)

- Resolves #17286
- Use the `download` attribute such that the browser will natively
initate a download dialog for the given URL.

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

options/locale/locale_en-US.ini

@@ -989,6 +989,7 @@ commit_graph.hide_pr_refs = Hide Pull Requests
 commit_graph.monochrome = Mono
 commit_graph.color = Color
 blame = Blame
+download_file = Download file
 normal_view = Normal View
 line = line
 lines = lines

templates/repo/view_file.tmpl

@@ -48,6 +48,7 @@
 				{{end}}
 				<a class="ui mini basic button" href="{{.RepoLink}}/commits/{{EscapePound .BranchNameSubURL}}/{{EscapePound .TreePath}}">{{.i18n.Tr "repo.file_history"}}</a>
 			</div>
+			<a download href="{{EscapePound $.RawFileLink}}"><span class="btn-octicon poping up" data-content="{{.i18n.Tr "repo.download_file"}}" data-position="bottom center" data-variation="tiny inverted">{{svg "octicon-download"}}</span></a>
 			{{if .Repository.CanEnableEditor}}
 				{{if .CanEditFile}}
 					<a href="{{.RepoLink}}/_edit/{{EscapePound .BranchName}}/{{EscapePound .TreePath}}"><span class="btn-octicon poping up" data-content="{{.EditFileTooltip}}" data-position="bottom center" data-variation="tiny inverted">{{svg "octicon-pencil"}}</span></a>