Adds tags to event-form

This commit adds a new form-element that allows tags to be added to an
event. Tags are inserted as comma-separated list. Currently no limit to
the number of tags is implemented
Adds Tags to the event-page

This commit adds the list of tags to the event-header
Convert comma-separated tags into array

This commit converts comma-separated lists into arrays before posting
them to the API.
Fixes phpcs-issue with missing whitespace
Conversion of tags field from comma separated list to an array doesn't need to be generalised until we have another field in the event form that is also a comma separated list.
Merge branch 'heiglandreas-feature/addTagsForEvent'