Fix for fwmark

Evidently, wireguard's (use of) fwmark is not well understood. In short,
it determines which routing table to use for a tunnel's packets.

Adding a fwmark to a roadwarrior client config won't do anything to the
actual packets sent to a peer: Packets do not get marked.

A QRCode with `FwMark = ...` in it is invalid.

FwMark is now excluded from client configs (but is written to the
server config /etc/wireguard/wgX.conf).

Potential breaking change of `WGUI_FORWARD_MARK` to `WGUI_FIREWALL_MARK`

But this has the effect of making users eventually notice that it
probably does not do what they want/think.

See:
https://ro-che.info/articles/2021-02-27-linux-routing

https://casavant.org/2020/10/10/wireguard-fwmark.html

https://www.blinkenlights.ch/ccms/posts/source-based-routing/

model/setting.go

@@ -10,7 +10,7 @@ type GlobalSetting struct {
 	DNSServers          []string  `json:"dns_servers"`
 	MTU                 int       `json:"mtu,string"`
 	PersistentKeepalive int       `json:"persistent_keepalive,string"`
-	ForwardMark         string    `json:"forward_mark"`
+	FirewallMark        string    `json:"firewall_mark"`
 	ConfigFilePath      string    `json:"config_file_path"`
 	UpdatedAt           time.Time `json:"updated_at"`
 }