Restructure Common module

[acelaya]

Rearrange Common module, splitting its content based on this criteria:

• IpGeolocation -> Feature/ip geolocation module #454
• Stuff which is trully common to any PHP web app (cache, logging, pagination, I18n, validation, utils...) -> Feature/common module #456
• Test utils (with base class for DB tests and API tests) -> Created TestUtils module #457
• Previews processing -> Created PreviewGenerator module #459

First two can be sent to separated composer libraries (shlinkio/shlink-common -> Shlinkio\Shlink\Common and shlinkio/shlink-ip-geolocation -> Shlinkio\Shlink\IpGeolocation)

The third one could also be a package of its own which shlink uses as a dev dependency.

Last one could be kept inside the main shlink project with different name and namespace (maybe Shlinkio\Shlink\PreviewGenerator?) and marked as deprecated to ease removing it for v2.