Display Benchmark hierarchies in the UI

[tobami]

(reported by Yaniv on [email protected])

The DB Schema allows to define a parent benchmark, but the UI has no concept of benchmark hierarchies, like in:

Benchmark X (parent)
  bandwidth
  moduleA
  moduleB

[str4d]

Ref: #120

[quasilyte]

Benchmark hierarchies can also help with high number of benchmarks display if individual trees can be collapsed.

By that, I mean this:

[+] FooBenchmarks (collapsed)
[-] BarBenchmarks (expanded)
  [-] BazBenchmarks
    * Baz1 (leaf)
    * Baz2
  * Bar1
  * Bar2
[+] QuxBenchmarks