Add a report: top flattened URLs #707

The current default Piwik behaviour works fine for most websites: categorizing pages into categories based on the / directory separator.
However when doing analytics, it is often very useful to find out in a glande what are the best actual URLs overall, rather than seeing an aggregate per directory.

We could build a report listing the best URLs overall, flattened.

eg. current report is (and will still exist as):

trac/      110 views
  index    10 views
  ticket/  60 views
     10    50 views
     5     10 views
  wiki/    40 views
     page1 35 views
     page2 5 views

The new flattened report would be:

trac/ticket/10  50 views
trac/wiki/page1 35 views
trac/index      10 views
trac/ticket/5   10 views
trac/wiki/page2 5 views

Each entry would link to the URL of the page.

To build this, we can either add this table to be processed at summarization (more disk usage and slower archiving, but faster stats querying), or process it in real time by loading all the data recursively from all tables/subtables in the Pages>Actions table and sort (less disk usage and no overhead at archiving, but potentially slow stats querying for a site with hundreds of sub categories, which leads to hundreds of select in the archive tables + unzip + load in memory + sort, etc.).