This adds a tab which displays system property and classpath information. This
can be useful in debugging various types of issues such as:

1. Extra/incorrect Hadoop jars being included in the classpath
2. Spark launching with a different JRE version than intended
3. Spark system properties not being set to intended values
4. User added jars that conflict with Spark jars

- Avoid exception when no tasks have finished for a stage
- Adding DOCTYPE so css renders properly
- Adding progress slider