Convert @mentions on Slack import. (#3837)

Converts @mentions in Slack imports for regular messages, comments and
Slack upload messages.

Updates the description on the Team Settings Import tab to remove
mention of @mentions not importing.

webapp/components/team_import_tab.jsx

@@ -48,7 +48,7 @@ class TeamImportTab extends React.Component {
             <div>
                 <FormattedHTMLMessage
                     id='team_import_tab.importHelp'
-                    defaultMessage="<p>To import a team from Slack go to Slack > Team Settings > Import/Export Data > Export > Start Export. Slack does not allow you to export files, images, private groups or direct messages stored in Slack. Therefore, Slack import to Mattermost only supports importing of text messages in your Slack team's public channels.</p><p>The Slack import to Mattermost is in 'Beta'. Slack bot posts do not yet import and Slack @mentions are not currently supported.</p>"
+                    defaultMessage="<p>To import a team from Slack go to Slack > Team Settings > Import/Export Data > Export > Start Export. Slack does not allow you to export files, images, private groups or direct messages stored in Slack. Therefore, Slack import to Mattermost only supports importing of text messages in your Slack team's public channels.</p><p>The Slack import to Mattermost is in 'Beta'. Slack bot posts do not yet import.</p>"
                 />
             </div>
         );

webapp/i18n/en.json

@@ -1664,7 +1664,7 @@
   "team_export_tab.unable": " Unable to export: {error}",
   "team_import_tab.failure": " Import failure: ",
   "team_import_tab.import": "Import",
-  "team_import_tab.importHelp": "<p>To import a team from Slack go to Slack > Team Settings > Import/Export Data > Export > Start Export. Slack does not allow you to export files, images, private groups or direct messages stored in Slack. Therefore, Slack import to Mattermost only supports importing of text messages in your Slack team's public channels.</p><p>The Slack import to Mattermost is in 'Beta'. Slack bot posts do not yet import and Slack @mentions are not currently supported.</p>",
+  "team_import_tab.importHelp": "<p>To import a team from Slack go to Slack > Team Settings > Import/Export Data > Export > Start Export. Slack does not allow you to export files, images, private groups or direct messages stored in Slack. Therefore, Slack import to Mattermost only supports importing of text messages in your Slack team's public channels.</p><p>The Slack import to Mattermost is in 'Beta'. Slack bot posts do not yet import.</p>",
   "team_import_tab.importSlack": "Import from Slack (Beta)",
   "team_import_tab.importing": " Importing...",
   "team_import_tab.successful": " Import successful: ",