rename pow section in settings to captcha and add options to configure

suggested difficulty factors for use in CAPTCHA configurations
estimates

The current CAPTCHA configuration panel requires the user to provide
difficulty factor <--> visitor threshold mapping, which can be tedious
if the user isn't familiar with those parameters. Also, it could lead to
ineffective limiting from mCaptcha's side, should it be configured
improperly.

So an estimate computed from well known statistics like peak, avg and
broke-my-site traffic could go a long way.

src/data.rs

@@ -103,7 +103,7 @@ impl SystemGroup {
 
     fn new_system<A: Save, B: MasterTrait>(m: Addr<B>, c: Addr<A>) -> System<A, B> {
         let pow = PoWConfigBuilder::default()
-            .salt(SETTINGS.pow.salt.clone())
+            .salt(SETTINGS.captcha.salt.clone())
             .build()
             .unwrap();
 
@@ -128,7 +128,7 @@ impl SystemGroup {
                 SystemGroup::Redis(captcha)
             }
             None => {
-                let master = EmbeddedMaster::new(SETTINGS.pow.gc).start();
+                let master = EmbeddedMaster::new(SETTINGS.captcha.gc).start();
                 let cache = HashCache::default().start();
                 let captcha = Self::new_system(master, cache);