Separate CSS file on build

[anders-kiaer]

This is a draft PR of how dash-core-components could build its static CSS into a separate .css file, which then is loaded through Dash's _css_dist.

Resolves #752, and enables e.g. running

import dash
import dash_html_components as html
import dash_core_components as dcc

from flask_talisman import Talisman

app = dash.Dash(__name__)

csp = {
    "default-src": "'self'",
    "script-src": [
        "'self'",
        # Due to https://github.com/plotly/dash/issues/630:
        "'sha256-jZlsGVOhUAIcH+4PVs7QuGZkthRMgvT2n0ilH6/zTM0='",
    ]
}

Talisman(app.server, content_security_policy=csp, force_https=False)

app.layout = html.Div(children=["Hello Dash!"])

if __name__ == "__main__":
    app.run_server()

[anders-kiaer]

Had to comment this one out temporarily in order to be able to run npm run build, but I guess someone knowing the details of WebpackDashDynamicImport would be able to figure out how to include it again.

[anders-kiaer]

Did another quick test, with this PR, to check if "inline style logic from Python" works with strict CSS CSP.

The result was that it works, as might be expected since Dash probably pass the style dictionaries from Python to React (which again sets the style attributes in a CSP compatible way with react >= 15).

import dash
import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output

from flask_talisman import Talisman

app = dash.Dash(__name__)

CSP = {
    "default-src": "'self'",
    "script-src": ["'self'", "'sha256-jZlsGVOhUAIcH+4PVs7QuGZkthRMgvT2n0ilH6/zTM0='"],
    "style-src": "'self'",
}

Talisman(app.server, content_security_policy=CSP, force_https=False)

app.layout = dcc.RadioItems(
    id="radioitems",
    options=[{"label": color, "value": color} for color in ["red", "blue", "green"]],
)

@app.callback(
    Output("radioitems", "style"), [Input("radioitems", "value")],
)
def _update_style(value):
    return {"backgroundColor": value}

if __name__ == "__main__":
    app.run_server()

Result: