Allow connecting to insecure (http) servers.

I'm a bit conflicted about this one because even though EteSync is
end-to-end encrypted, people should be using TLS.
I can't think of a reason why anyone would want to use HTTP over HTTPS,
especially given let's encrypt exists and EteSync supports self-signed
certs, but I guess it's better to allow it for now, at least until we
have a nice error message for it.

Fixes #81

app/src/main/res/xml/network_security_config.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <network-security-config>
-    <base-config>
+    <base-config cleartextTrafficPermitted="true">
         <trust-anchors>
             <certificates src="system"/>
             <certificates src="user"/>