Update notice: a newer release shows in the web interface

Once a day the server asks Gitea or GitHub, as picked under Settings ->
Updates, for the latest release. A newer one shows as a pill in the
sidebar, a banner on the Dashboard and in the Updates card with its
release notes and the commands to update this server. Drafts and
pre-releases are ignored, nothing about the server is sent, and the check
can be switched off. POST /updates/check checks now.

main_test.go

@@ -78,6 +78,7 @@ func TestValidate(t *testing.T) {
 		"bad dns":         func(c *Config) { c.Peers[0].DNS = []string{"dns.example"} },
 		"bad port":        func(c *Config) { c.Server.ListenPort = 70000 },
 		"unmasked net":    func(c *Config) { c.Server.IPv4 = "10.84.12.5/24" },
+		"update source":   func(c *Config) { c.Updates.Source = "sourceforge" },
 	} {
 		cc := c.clone()
 		mutate(cc)