6627: Add Chouette favicon

Disable the checkboxes next to pending referentials. This prevents them
from being deleted by the batch deletion operation.

In general it seems like we shouldn't be operating on pending
referentials, so if any new batch actions are added, it would make sense
to me for the checkboxes to stay disabled.

We want to disallow deletion of pending referentials because this can
put the data in an inconsistent state that requires manual intervention
to clean (I think).

Refs #6858

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