fix: enable translating login form placeholder

src/components/login_form/login_form.vue

@@ -8,7 +8,7 @@
       <form v-on:submit.prevent='submit(user)' class='login-form'>
         <div class='form-group'>
           <label for='username'>{{$t('login.username')}}</label>
-          <input :disabled="loggingIn" v-model='user.username' class='form-control' id='username' placeholder='e.g. lain'>
+          <input :disabled="loggingIn" v-model='user.username' class='form-control' id='username' v-bind:placeholder="$t('login.placeholder')">
         </div>
         <div class='form-group'>
           <label for='password'>{{$t('login.password')}}</label>

src/i18n/messages.js

@@ -314,6 +314,7 @@ const en = {
   login: {
     login: 'Log in',
     username: 'Username',
+    placeholder: 'e.g. lain',
     password: 'Password',
     register: 'Register',
     logout: 'Log out'