Added new option to download .zuliprc file directly from settings
page. This should help reduce friction when setting up new
bots/integrations. This new feature is available in the bot cards and
the 'show your API key' section. One caveat is that the filename is
automatically set to 'zuliprc' instead of '.zuliprc', since as most
browsers do not allow filenames to start with a dot.
Fixes#2327.