LIVY-353. Web UI. Added link to Spark UI for Sessions running on Yarn

[ajbozarth]

LIVY-353

Followup to #319 adding a link to the Application Id that links to the Spark UI. The link is grabbed from session.appInfo.sparkUiUrl if it exists, otherwise no link is created.

Screenshots

With an appId without link (in this case before the UI is generated):

With an appId link:

What it links to:

[codecov-io]

Codecov Report

Merging #327 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #327      +/-   ##
============================================
- Coverage     70.67%   70.61%   -0.06%     
+ Complexity      716      715       -1     
============================================
  Files            96       96              
  Lines          5084     5084              
  Branches        746      746              
============================================
- Hits           3593     3590       -3     
- Misses          992      997       +5     
+ Partials        499      497       -2

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/cloudera/livy/rsc/rpc/RpcDispatcher.java	66.3% <0%> (-3.27%)	19% <0%> (-1%)

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[ajbozarth]

@zjffdu could you take a look at this and #326 since you requested these additions

[ajbozarth]

@jerryshao would you also review this as well?

[jerryshao]

Sorry @ajbozarth for late response. The patch LGTM.

[jerryshao]

Hi @ajbozarth , sorry for the delay, can you submit this PR against https://github.com/apache/incubator-livy, thanks.

[ajbozarth]

moved to apache/incubator-livy#4