[web-ui] KUDU-1588 /tablets page should separate out tombstoned tablets to a separate section
[web-ui] KUDU-1588 /tablets page should separate out tombstoned tablets to a separate section

- Separated tombstoned and live tablet tables in /tablets ui page.
- Added one sentence explaining tombstoned tablets as:
   "Tombstoned tablets are tablets that previously stored a replica on this server."
- Tombstoned tablets table is displayed only when they are found.

Change-Id: I0aa9b67749234c5d68899a82d6b1493633bb78c5
Reviewed-on: http://gerrit.cloudera.org:8080/4526
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

InspurUSA/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,helifu/kudu,cloudera/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu