Replace snowplow_web with a base that can be compatible with mobile events

[matus-tomlein]

Is your feature request related to a problem? Please describe.

Prepare to add support for mobile events.

Describe the solution you'd like

Take out the snowplow_web package as the base and use a custom one taken from the ecommerce package. The motivation for this is to be able to process mobile events as well.

Additional context

We won't yet support mobile events in this issue. We can only generate mobile events from the new media schemas which are not yet supported by the package, so we will add support for mobile later, when adding support for the new media schemas. This issue just prepares for this in advance.