Generate css from plugin scss on build

[finanalyst]

Plugins typically contain SCSS files from which CSS is generated. Normally the CSS is generated by a developer to test the plugin. Sometimes this does not happen. The CSS is then not the intended one. This occurred for a style change to the Programs tab.
There is a bash utility in this repo called update-css, which generates the CSS from SCSS on a per-plugin basis.
I suggest that part of the build process should be to run update-css. If this is a good idea, then

1. update-css should be changed to ensure that all plugins are checked if no argument is provided to it
2. dart scss needs installing in the build environment (as documented in the README)
3. update-css is run each build (dart scss does not change anything if the css is more recent than the scss).

@dontlaugh is this reasonable?