Split UI from benchmark runner

[datapythonista]

asv has (at least) two different components:

• The code to run benchmarks (e.g. asv run...)
• A UI to visualize the results of benchmarks (e.g. https://pandas.pydata.org/speed/pandas/)

We'd like to eventually have the asv UI in a different repo, and make the benchmark runner work with other UI's, like codebench or codespeed. For now, we can start by moving all the UI code to a separate module/directory in this repo