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>

modules/structs/repo_file.go

@@ -50,6 +50,14 @@ type UpdateFileOptions struct {
 	FromPath string `json:"from_path" binding:"MaxSize(500)"`
 }
 
+// ApplyDiffPatchFileOptions options for applying a diff patch
+// Note: `author` and `committer` are optional (if only one is given, it will be used for the other, otherwise the authenticated user will be used)
+type ApplyDiffPatchFileOptions struct {
+	DeleteFileOptions
+	// required: true
+	Content string `json:"content"`
+}
+
 // FileLinksResponse contains the links for a repo's file
 type FileLinksResponse struct {
 	Self    *string `json:"self"`