implement support for HTTP proxy (#489)

This change allows the user to manually enter an unauthenticated proxy
configuration to be used for all API connections. This is mainly
intended for using Tusky with Tor (via Orbot or a local proxy).

app/src/main/java/com/keylesspalace/tusky/BaseActivity.java

@@ -137,8 +137,10 @@ public abstract class BaseActivity extends AppCompatActivity {
                 .registerTypeAdapter(Spanned.class, new SpannedTypeAdapter())
                 .create();
 
+        SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
+
         OkHttpClient.Builder okBuilder =
-                OkHttpUtils.getCompatibleClientBuilder()
+                OkHttpUtils.getCompatibleClientBuilder(preferences)
                         .addInterceptor(new AuthInterceptor(this))
                         .dispatcher(mastodonApiDispatcher);