[super] Search UX improvements

[shevski]

1. Make it possible to search by tag (e.g. by typing tag:csv into the search bar and clicking enter, it should add the 'csv' tag facet to the search)

2. Rename and standardise the list of format tags, on search page this should also be called 'Format' instead of 'res_format' (in the right hand side bar on search page).

3. Make it possible to view full list of tags, formats and groups by clicking on the name. From here you should be table to click on a classification and go back to a search page faceted by that classification. E.g. from search page, click on 'tags', on tag page click on 'london' or whatever, and be navigated back to search page with search within 'london' tag only. Or y'know, a better way of doing it.

4. More standard classifications, such as 'Location' and 'Theme' - like on publicdata.eu

5. Blue search button should be displayed in line with the search bar, not underneath

6. Datasets should be displayed in order of most viewed or downloaded instead of alphabetically. For alphabetic search we could consider adding a way to facet by first letter of dataset name

[zephod]

1. Already exists; "tags:csv" rather than "tag:csv"
2. Format is user selected; we just show the most popular choices. Perhaps an effort should be made to force people to homogenize their format choices (my work with automatic icons will do this to an extent). Also, res_format doesn't appear on thedatahub.org, but for some reason it does on test...
3. There are thousands. We just show the most popular/relevant tags for your search. This could break out into another ticket though - eg. autocompleting tag input on the page?
4. Can't remember what this is...
5. I did this a while ago and popular opinion was that it should change back. This gives us room for expanded options like on thedatahub's live search page right now, and matches the layout of the Groups view page.
6. They should maybe be ordered by relevance, which includes popularity in some metric? This should be broken into a seperate ticket if you want it to go ahead and given to a Solr expert.