[SPARK-7378] [CORE] Handle deep links to unloaded apps.

The code was treating deep links as if they were attempt IDs, so
for example if you tried to load "/history/app1/jobs" directly,
that would fail because the code would treat "jobs" as an attempt id.

This change modifies the code to try both cases - first without an
attempt id, then with it, so that deep links are handled correctly.
This assumes that the links in the Spark UI do not clash with the
attempt id namespace, though, which is the case for YARN at least,
which is the only backend that currently publishes attempt IDs.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5922 from vanzin/SPARK-7378 and squashes the following commits:

96f648b [Marcelo Vanzin] Fix comparison.
ed3bcd4 [Marcelo Vanzin] Merge branch 'master' into SPARK-7378
23483e4 [Marcelo Vanzin] Fat fingers.
b728f08 [Marcelo Vanzin] [SPARK-7378] [core] Handle deep links to unloaded apps.