[Draft] Add option of subscribing to dataset changes

Closes #7

added 8 commits

• 680998c3...675de373 - 7 commits from branch drupal-to-ckan
• 7dfa0bdd - Add button for following dataset

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added 1 commit

• 7e2b79e7 - Add button for following dataset

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It turns out that Activity table is empty as package actions are not connected to activity model in current version of ckan core. So subscriptions at this point don't make sense, neither does our implementation, as we plan to move to the latest ckan version.

When we'll move to the latest version of ckan, there is ckanext-activity plugin that has all the logic, so I'm leaving this here for that time. If this commit will still make sense great, otherwise just delete this branch and close the MR.

closed