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

[ajbozarth]

PR moved from old repo cloudera/livy#327

LIVY-353

Followup to cloudera/livy#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 #4 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master       #4      +/-   ##
============================================
+ Coverage     70.59%   70.71%   +0.11%     
+ Complexity      730      729       -1     
============================================
  Files            96       96              
  Lines          5153     5153              
  Branches        776      776              
============================================
+ Hits           3638     3644       +6     
+ Misses          999      992       -7     
- Partials        516      517       +1

Impacted Files	Coverage Δ	Complexity Δ
...n/java/com/cloudera/livy/rsc/driver/RSCDriver.java	76.29% <0%> (-1.3%)	39% <0%> (-2%)
...in/java/com/cloudera/livy/rsc/ContextLauncher.java	83.08% <0%> (+2.48%)	19% <0%> (ø)	⬇️
...m/cloudera/livy/repl/SparkContextInitializer.scala	63.15% <0%> (+3.5%)	0% <0%> (ø)	⬇️
...va/com/cloudera/livy/rsc/rpc/KryoMessageCodec.java	98.11% <0%> (+3.77%)	19% <0%> (+1%)	⬆️

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[jerryshao]

LGTM, merging to master.