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_ios_app/get_ios_app.jsx

@@ -0,0 +1,68 @@
+// 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 AppStoreButton from 'images/app-store-button.png';
+import IPhone6Mockup from 'images/iphone-6-mockup.png';
+
+export default class GetIosApp extends React.Component {
+    render() {
+        return (
+            <div className='get-app get-ios-app'>
+                <h1 className='get-app__header'>
+                    <FormattedMessage
+                        id='get_app.iosHeader'
+                        defaultMessage='Mattermost works best if you switch to our iPhone app'
+                    />
+                </h1>
+                <hr/>
+                <a
+                    className='get-ios-app__app-store-link'
+                    href={global.window.mm_config.IosAppDownloadLink}
+                    rel='noopener noreferrer'
+                >
+                    <img src={AppStoreButton}/>
+                </a>
+                <img
+                    className='get-app__screenshot'
+                    src={IPhone6Mockup}
+                />
+                <h2 className='get-ios-app__already-have-it'>
+                    <FormattedMessage
+                        id='get_app.alreadyHaveIt'
+                        defaultMessage='Already have it?'
+                    />
+                </h2>
+                <a
+                    className='btn btn-primary get-ios-app__open-mattermost'
+                    href='mattermost://'
+                >
+                    <FormattedMessage
+                        id='get_app.openMattermost'
+                        defaultMessage='Open Mattermost'
+                    />
+                </a>
+                <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>
+        );
+    }
+}