Add missing strings for translation

1. Explore page header
2. “by” in the project thumbnail

src/components/thumbnail/thumbnail.jsx

-var React = require('react');
 var classNames = require('classnames');
+var FormattedMessage = require('react-intl').FormattedMessage;
+var React = require('react');
 
 require('./thumbnail.scss');
 
@@ -32,7 +33,10 @@ var Thumbnail = React.createClass({
         if (this.props.creator) {
             extra.push(
                 <div key="creator" className="thumbnail-creator">
-                    by <a href={'/users/' + this.props.creator + '/'}>{this.props.creator}</a>
+                    <FormattedMessage id={'thumbnail.by'} />{' '}
+                    <a href={'/users/' + this.props.creator + '/'}>
+                        {this.props.creator}
+                    </a>
                 </div>
             );
         }

src/l10n.json

@@ -154,5 +154,7 @@
     "registration.waitForApprovalDescription": "You can log into your Scratch Account now, but the features specific to Teachers are not yet available. Your information is being reviewed. Please be patient, the approval process can take up to 24 hours. You will receive an email indicating your account has been upgraded once your account has been approved.",
     "registration.welcomeStepDescription": "You have successfully set up a Scratch account! You are now a member of the class:",
     "registration.welcomeStepPrompt": "To get started, click on the button below.",
-    "registration.welcomeStepTitle": "Hurray! Welcome to Scratch!"
+    "registration.welcomeStepTitle": "Hurray! Welcome to Scratch!",
+
+    "thumbnail.by": "by"
 }

src/views/explore/explore.jsx

@@ -111,10 +111,12 @@ var Explore = injectIntl(React.createClass({
         );
     },
     render: function () {
+        var formatMessage = this.props.intl.formatMessage;
+
         return (
             <div>
                 <div className='outer'>
-                    <Box title={'Explore'}>
+                    <Box title={formatMessage({id: 'general.explore'})}>
                         <SubNavigation className='categories'>
                             {this.getBubble('all')}
                             {this.getBubble('animations')}