http: add "Use reverse proxy host" option

added "Use reverse-proxy reported "Host" when possible" option, which
will prefer using the "Host" or "X-Forwarded-Host" values instead of
"External jfa-go URL" in the web app. To do so, app.ExternalDomain/URI
are now functions which take *gin.Context (the latter optionally). The
protocol for the request is determined from X-Forwarded-Proto(col), so
make sure your proxy includes it.

The wiki will have been updated to mention the new option.

pwreset.go

@@ -30,7 +30,7 @@ func (app *appContext) GenInternalReset(userID string) (InternalPWR, error) {
 
 // GenResetLink generates and returns a password reset link.
 func (app *appContext) GenResetLink(pin string) (string, error) {
-	url := app.ExternalURI
+	url := app.ExternalURI(nil)
 	var pinLink string
 	if url == "" {
 		return pinLink, errors.New(lm.NoExternalHost)