config: migrate "url_base" dupes to "jfa_url"

URL Base now refers to JUST the subfolder portion, i.e. `/accounts` if
you access jfa-go at `http://jellyf.in/accounts`. General > "jfa_url"/"External
jfa-go URL" now refers to the WHOLE URL you access jfa-go at, i.e.
`http://jellyf.in/accounts`. The settings in "invite emails" and
"password resets" have been removed, and a value chosen from the two
applied to "jfa_url". Migration also makes a config backup. Adds a
"deprecated" flag to config-base, which just tells the UI to not show
it (for now). Also added some warnings related to the URL base /
External URL.

views.go

@@ -740,11 +740,7 @@ func (app *appContext) InviteProxy(gc *gin.Context) {
 	discord := discordEnabled && app.config.Section("discord").Key("show_on_reg").MustBool(true)
 	matrix := matrixEnabled && app.config.Section("matrix").Key("show_on_reg").MustBool(true)
 
-	userPageAddress := app.config.Section("invite_emails").Key("url_base").String()
-	if userPageAddress == "" {
-		userPageAddress = app.config.Section("password_resets").Key("url_base").String()
-	}
-	userPageAddress += "/my/account"
+	userPageAddress := fmt.Sprintf("%s/my/account", app.ExternalHost)
 
 	fromUser := ""
 	if invite.ReferrerJellyfinID != "" {