Added toggle for download messages

When room is locked there is no UI to indicate that you cannot download
messages #1055

* Added new download icons
* Added toggleDownloadButton to Chat.ui.js
* Updated room methods to change button status on room change and lock
  change
Added keyboard support for paste upload dialog

Added support for <esc> and <enter> on the preview dialog for pasting images

* ESC: closes the dialog and stopping upload
* ENTER: uploads the file and closes dialog

Also focus to the chat box after closing.
Merge pull request #1091 from Buildstarted/dev

Added toggle for download messages/paste images additions