Support so print can print only important text.
SVG modified to use display:inline/none based on overlay True/False and
a symlink to make it easy to change to a completely seperate svg if that
is not optimal.
SVG added layers to split text and graphic so graphics can be easily
hidden and printed.
Closes: #38
* Switch to showing the PNG version by default, as this reflects what
will actually be rendered and sent to the printer
* Include the greyscale filter
* Include the twemoji font we'll use for rendering the badges
* Add a greyscale filter to text for more accurate preview
* Always default to SVG preview as it's the most accurate (cairo
doesn't do a great job of handling custom fonts when it converts to
png/pdf)
* Always use roboto font.
* Undo some of the debugging done early in this series of patches
* Add ability for a user to preview their own badge
* Add a template for the LCA2018 badge
"Next" is green, indicating that it's the default path, the way
forward. "Back" is available but blue.
For extra consistency, the initial "Get ticket" button is now also a btn-success
