[feature] Implement static minification and cache invalidation

[nemesifier]

We need implement static asset minification and make sure Javascript and CSS files get updated automatically without having to ask users to refresh their browser cache.

Unfortunately django-pipeline does not support simple minification without bundling files together, which requires changes to the code of OpenWISP modules which are overkill (given also HTTP2 multiplexing it's not extremely needed).

But its feature to Minify HTML is indeed useful, so we can use a mixture of different tools.

Packages to install: pip install django-pipeline django-compress-staticfiles

Configuration example:

# HTML minification with django pipeline
INSTALLED_APPS.append('pipeline')
MIDDLEWARE.append('pipeline.middleware.MinifyHTMLMiddleware')
PIPELINE = {
    'PIPELINE_ENABLED': True
}

# static files minification and invalidation with django-compress-staticfiles
STATICFILES_STORAGE = 'compress_staticfiles.storage.CompressStaticFilesStorage'
# GZIP compression is handled by nginx
BROTLI_STATIC_COMPRESSION = False
GZIP_STATIC_COMPRESSION = False

When using this configuration, a bug in openwisp modules is surfacing, we should not use the static() function in admin class media definitions, hence this has to be fixed/checked in the following modules:

• django-x509 (openwisp/django-x509@264cfb8)
• django-loci
• openwisp-utils
• openwisp-users
• openwisp-controller (openwisp/openwisp-controller@235ec6f)
• openwisp-network-topology ([fix] Removed static() call from media assets openwisp-network-topology#96)
• openwisp-radius ([fix] Removed static() call from media assets openwisp-radius#187)
• openwisp-monitoring
• openwisp-ipam
• openwisp-firmware-upgrader (openwisp/openwisp-firmware-upgrader@be3a6bb)