Update Contributing.md and Demos to match current repo structure

[HammadTheOne]

Some parts of the CONTRIBUTING.md require updates to bring it in line with the currently applied process in the dash-bio repo. Alternatively, the directories in the repo that don't comply with the contributing guidelines can be updated to match best practices. For example the following instructions specify snake_case, but the sub-directories' names are hyphenated and include an app.py.

In the `tests/dashbio_demos/` subfolder, please create a file named
`app_{your component name in snake case}.py`. In this file, please include the

Similarly, the following subfolder and (I believe) the gallery structure no longer exist.

In the `tests/dashbio_demos/images/` subfolder, please include a PNG file named
`pic_{your component name in snake case}.png`.

[HammadTheOne]

As part of this housekeeping issue, we can also address some changes required to run the dashbio_demos template apps locally after the repo was restructured in #459.

It includes the following points of interest:

• Update individual app requirements to fix dependency issues (eg. sk-learn).
• Restructure layout_helper so the module is accessible either as a absolute import or relative import.
• Update instructions in CONTRIBUTING.md for updated process.
• Fix CSS issues with dcc.Loading class (Graph dimension problem with dcc.Loading — regression (?) in DCC 1.9.1 dash-core-components#831 (comment))