Fix PreUp, PostUp, PreDown, and PostDown

* Escaping HTML in several places.
* Adds PreUp config when one didn't exist.
* Adds environment variable support for PreUp and PreDown.

closes #549
closes #655
closes #656

See also
--------

- https://github.com/samrocketman/addons-homeassistant/issues/9

Co-authored-by: Robert Willert <rwillert@users.noreply.github.com>

model/server.go

@@ -22,6 +22,7 @@ type ServerInterface struct {
 	Addresses  []string  `json:"addresses"`
 	ListenPort int       `json:"listen_port,string"` // ,string to get listen_port string input as int
 	UpdatedAt  time.Time `json:"updated_at"`
+	PreUp      string    `json:"pre_up"`
 	PostUp     string    `json:"post_up"`
 	PreDown    string    `json:"pre_down"`
 	PostDown   string    `json:"post_down"`