Update general template's header preview and subscribe variables

[heyitsolivia]

These variables need to be updated to make sense in the context of the general template.

    "previewText": "Our daily a.m. roundup of #opengov news",
    "subscribeLink": "http://eepurl.com/bkdY1P"

Note: subscribeLink may be removed since the general template is not using the header-subscribe partial where it would have been used.

[heyitsolivia]

@linzjax should have generic general preview text for this

[heyitsolivia]

Also Evan may have some input about the preview text.