Add apply-patch, basic revert and cherry-pick functionality (#17902)

This code adds a simple endpoint to apply patches to repositories and
branches on gitea. This is then used along with the conflicting checking
code in #18004 to provide a basic implementation of cherry-pick revert.

Now because the buttons necessary for cherry-pick and revert have 
required us to create a dropdown next to the Browse Source button
I've also implemented Create Branch and Create Tag operations.

Fix #3880 
Fix #17986 

Signed-off-by: Andrew Thornton <art27@cantab.net>

templates/repo/home.tmpl

@@ -89,6 +89,11 @@
 								{{.i18n.Tr "repo.editor.upload_file"}}
 							</a>
 						{{end}}
+						{{if .CanAddFile}}
+							<a href="{{.RepoLink}}/_diffpatch/{{.BranchName | PathEscapeSegments}}/{{.TreePath | PathEscapeSegments}}" class="ui button">
+								{{.i18n.Tr "repo.editor.patch"}}
+							</a>
+						{{end}}
 					{{end}}
 					{{if and (ne $n 0) (not .IsViewFile) (not .IsBlame) }}
 						<a href="{{.RepoLink}}/commits/{{.BranchNameSubURL}}/{{.TreePath | PathEscapeSegments}}" class="ui button">