Use a transparent background version of the favicon instead of setting
it on white as per the new requirements.

Include both 16x16 and 32x32 sizes to support retina displays.

I resized the `Logo_Chouette-256x256 transparent.png` logo in GIMP to
16x16 and 32x32 sizes using cubic interpolation. Then used Imagemagick
to create the ICO file:

    $ convert \
        Logo_Chouette-16x16\ transparent.png \
        Logo_Chouette-32x32\ transparent.png \
        favicon.ico

Refs #6627

Adds a favicon to the site to respond to these errors:

    ActionController::RoutingError (No route matches [GET] "/favicon.ico"):

Using `favicon_link_tag` allows us to get the image from the asset
pipeline, which makes it easier to update if necessary, since browsers
tend to cache the default `/favicon.ico` indeterminably (source:
http://api.rubyonrails.org/classes/ActionView/Helpers/AssetTagHelper.html#method-i-favicon_link_tag).

According to some Stack Overflow posts I've been reading, it looks like
it's advisable to put the `favicon.ico` file in both `/assets/images/`
and `/public/`:
- https://stackoverflow.com/questions/15687506/actioncontrollerroutingerror-no-route-matches-get-favicon-ico-in-rails#15688041
- https://stackoverflow.com/questions/11071445/favicon-in-ruby-on-rails-application

This way, you can serve the favicon from the asset pipeline, but if any
requesters try to get `/favicon.ico`, the server will respond with the
image instead of an error.

To be honest, when I was testing locally, I couldn't get the icon in
`/public` to display, both without a `link` tag and with the following
`link` tag:

    / application.html.slim
    / ...
    link href='/favicon.ico' rel='shortcut icon'

As a result, I'm adding the image to `/public` with fingers crossed that
it's going to work if requested.

I generated the `favicon.ico` file with a 256x256px PNG supplied by
Cyrielle in the Redmine ticket and the following ImageMagick command:

    $ convert -resize x16 -gravity center -crop 16x16+0+0 \
        Logo_Chouette-256x256.png -flatten -colors 256 favicon.ico

(Copied from:
https://stackoverflow.com/questions/3185677/converting-gifs-pngs-and-jpgs-to-ico-files-using-imagemagick)

Refs #6627