PLT-3640 Add mobile landing pages (#3674)

* PLT-3640 Moved all clientside user agent snooping into a single file

* PLT-3640 Added mobile landing pages on login to iOS and Android web apps

* PLT-3640 Moved landing page to appear before first login

* PLT-3640 Fixed detection of Chrome on Android

* PLT-3640 Disabled mobile landing pages when their respective URLs are set to blank

webapp/components/get_android_app/get_android_app.jsx

@@ -0,0 +1,75 @@
+// Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import React from 'react';
+
+import {FormattedMessage} from 'react-intl';
+import {Link} from 'react-router';
+
+import MattermostIcon from 'images/favicon/android-chrome-192x192.png';
+import Nexus6Mockup from 'images/nexus-6p-mockup.png';
+
+export default class GetAndroidApp extends React.Component {
+    render() {
+        return (
+            <div className='get-app get-android-app'>
+                <h1 className='get-app__header'>
+                    <FormattedMessage
+                        id='get_app.androidHeader'
+                        defaultMessage='Mattermost works best if you switch to our Android app'
+                    />
+                </h1>
+                <hr/>
+                <div>
+                    <img
+                        className='get-android-app__icon'
+                        src={MattermostIcon}
+                    />
+                    <div className='get-android-app__app-info'>
+                        <span className='get-android-app__app-name'>
+                            <FormattedMessage
+                                id='get_app.androidAppName'
+                                defaultMessage='Mattermost for Android'
+                            />
+                        </span>
+                        <span className='get-android-app__app-creator'>
+                            <FormattedMessage
+                                id='get_app.mattermostInc'
+                                defaultMessage='Mattermost, Inc'
+                            />
+                        </span>
+                    </div>
+                </div>
+                <a
+                    className='btn btn-primary get-android-app__continue'
+                    href={global.window.mm_config.AndroidAppDownloadLink}
+                >
+                    <FormattedMessage
+                        id='get_app.continue'
+                        defaultMessage='Continue'
+                    />
+                </a>
+                <img
+                    className='get-app__screenshot'
+                    src={Nexus6Mockup}
+                />
+                <span className='get-app__continue-with-browser'>
+                    <FormattedMessage
+                        id='get_app.continueWithBrowser'
+                        defaultMessage='Or {link}'
+                        values={{
+                            link: (
+                                <Link to='/switch_team'>
+                                    <FormattedMessage
+                                        id='get_app.continueWithBrowserLink'
+                                        defaultMessage='continue with browser'
+                                    />
+                                </Link>
+                            )
+                        }}
+                    />
+                </span>
+            </div>
+        );
+    }
+}