account lookup (#85)

-implemented /api/v1/accounts/lookup
-prevents user from creating an account with a taken username
-prevents the previous HTTP 500 error

server/src/main/java/edu/sjsu/moth/server/controller/AppController.java

@@ -112,6 +112,13 @@ public Mono<ResponseEntity<Object>> registerAccount(@RequestHeader("Authorizatio
                 .map(token -> ResponseEntity.ok(new TokenResponse(token.token, "*")));
     }
 
+    @GetMapping("/api/v1/accounts/lookup")
+    public Mono<ResponseEntity<Account>> lookUpAccount(@RequestParam String username) {
+        return accountService.getAccount(username)
+                .map(ResponseEntity::ok)
+                .defaultIfEmpty(ResponseEntity.notFound().build());
+    }
+
     @PostMapping("/api/v1/emails/confirmations")
     String emailsConfirmations() {
         // we don't use email verification... YET!