Update build chain for dash 0.32.1

[pbugnion]

As of release 0.32.1, dash prefers building the Python classes at build time rather than at run time. Dash comes with a new utility, dash-generate-components, that builds the Python classes. This PR refactors the tooling to use this.

Now, running npm install populates dash_bootstrap_components/_components with the Python class files containing the components. These then get built into the source distribution.

I have deliberately diverged from some of the recommendations outlined in issue #93 :

• The build artefacts are not source controlled. This follows the convention we've adopted in dbc so far.
• Generated components are segregated to a _components subdirectory, rather than being in the dash_bootstrap_components root. This makes it easier to clearly differentiate Python files that are written manually (e.g. dbc.themes, __init__.py, the upcoming helper to generate a table from a dataframe) from auto-generated source.

At the moment, running npm install shows some tracebacks when react-docgen tries to parse the test files. I think this is an issue that needs to be fixed in dash, not directly in here. I have raised an issue to that effect. This doesn't actually stop us from using the build artefacts, it just makes the build process confusing.

I have released this as 0.2.4-alpha1 to test it and everything seems to be working. Nevertheless, since this is a fairly tricky change, it'd be great to:

• check that you can actually run npm i (and then maybe run the documentation or some other app)
• check that you can install and run something off 0.2.4-alpha1 with pip install dash-bootstrap-components==0.2.4-alpha1.

Addresses issue #93.

[tcbegley]

Shall we add something like black dash_bootstrap_components/_components to this? Otherwise the generated Python files are going to fail the build once they get added to version control.

Actually ignore this, I think we're not keeping these under version control right? We're just generating them for inclusion in the distribution?

[pbugnion]

Yes, we don't keep the generated sources under version control (unlike, for instance, dash-core-components).

While we could still reformat them with black, I'd vote against because:

• it makes building a little bit longer for no benefit
• it adds more point of failures in the build chain.

[tcbegley]

Looks great @pbugnion! Thanks for looking into this and setting everything up.

I have testing building, running the demo, and installing 0.2.4-alpha1 and all worked for me.

I made a comment inline, but have since thought that actually it's not relevant, and this is all fine as is.