HUE-1811 [search] Sample collections

Create 3 examples of collections with several facets and features:
- twitter
- yelp
- log search

The index page of the collections works even without a valid solr for demo purpose.
The collections are moved from the search dropdown to the main menu bar.
Some sample data is available for filling a reald index. Some basic indexing scripts
are in progress in order to offer a good quick start experience.

We are using hardoced ids in the fixtures to keep it simple.

Todo:
- click on install examples installs the example
- add more data and finish the indexing scripts
- collection level permissions
- alert message when using a demo collection without a valid solr
HUE-1811 [search] Alert message when using a demo collection without a valid Solr
HUE-1811 [search] Install examples button in quick start wizard

Also display a landing page when no collections are setup
HUE-1811 [search] Add script for creating the Solr collections

Renamed the example collection to *_demo
[search] Adding sample icons
[search] Integrating icons for demo collections