Misc/event registration split

## Ready State: Yes

This pull request is related to the following JIRA tickets: https://imagex.atlassian.net/browse/LDC-154

- Event Registration fields moved to a separate sub events feature (imagex_events_registration)
- Remove field_offline_url textfield & replaced with a link field (common fields field_link won't show in existing fields list)

## Requirements & Dependencies

None

## Deployment Instructions

* Merge code.
* Clear caches

## Usage/Test Instructions

* Manually delete "field_offline_url", "field_registration" & "field_registration_type" as features won't remove them
* Create a new event and ensure the new "Event External Link" shows
* Ensure that enabling the "imagex_events_registration" feature brings back the  "field_registration" & "field_registration_type" fields