Refactor Cron and merge dashboard tasks (#10745)

* Refactor Cron and merge dashboard tasks

* Merge Cron and Dashboard tasks
* Make every cron task report a system notice on completion
* Refactor the creation of these tasks
* Ensure that execution counts of tasks is correct
* Allow cron tasks to be started from the cron page

* golangci-lint fixes

* Enforce that only one task with the same name can be registered

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

* fix name check

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

* as per @guillep2k

* as per @lafriks

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

* Add git.CommandContext variants

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

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

modules/auth/admin.go

@@ -51,7 +51,7 @@ func (f *AdminEditUserForm) Validate(ctx *macaron.Context, errs binding.Errors)
 
 // AdminDashboardForm form for admin dashboard operations
 type AdminDashboardForm struct {
-	Op int `binding:"required"`
+	Op string `binding:"required"`
 }
 
 // Validate validates form fields