[openstreetmap/openstreetmap-website] Add support for rotating banners, add banners for SOTM, SOTMUS (#1240)

[Bryan Housel]

This PR adds support for rotating OSM event banners in the sidebar

* Dismissing the banner will set a cookie so it won't be shown again.
* Cookie tracks how recently the banner was shown, so they will rotate if multiple are active
* Banners are enabled in `_banner.html.erb` 
* Images go in `app/assets/images/banners/`
* This PR also adds banners for SOTM-US 2016 in Seattle and SOTM 2016 in Brussels

```erb
<% active_banners = {
  :sotmus2016 => {
    :alt => 'State of the Map US 2016',
    :link => 'http://stateofthemap.us/',
    :img => 'banners/sotmus-2016.jpg'
  },
  :sotm2016 => {
    :alt => 'State of the Map 2016',
    :link => 'http://2016.stateofthemap.org/',
    :img => 'banners/sotm-2016.jpg'
  }
}
```

cc @lxbarth @tatsvc @cliffordsnow @iandees 

pretty picture:

<img width="1099" alt="screenshot 2016-06-20 18 18 45" src="https://cloud.githubusercontent.com/assets/38784/16212087/7bb2f446-3713-11e6-9e98-460de10a6a69.png">

You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/1240

-- Commit Summary --

  * Add support for rotating banners, add banners for SOTM, SOTMUS

-- File Changes --

    A app/assets/images/banners/.keep (0)
    A app/assets/images/banners/sotm-2016.jpg (0)
    A app/assets/images/banners/sotmus-2016.jpg (0)
    M app/assets/javascripts/index.js (18)
    M app/assets/stylesheets/common.scss (44)
    A app/views/layouts/_banner.html.erb (41)
    M app/views/layouts/map.html.erb (7)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/1240.patch
https://github.com/openstreetmap/openstreetmap-website/pull/1240.diff

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