Investigate: Support for events on PersonalSnippets

We want to get an idea of the amount of work needed to display activity related to PersonalSnippets in the user's profile (along with ProjectSnippets activity). As per Fran's suggestion, we could achieve that by:

• Removing this return in Notes::PostProcessService
• Including snippets with project_id: nil in this query from Gitlab::ContributionsCalendar. Possibly, trying that in Gitlab::ContributionsCalendar#events_by_date and UserRecentEventsFinder#execute as well
• Testing related views like calendar_activities to help identify other changes

Context

• Currently, only comments on ProjectSnippets are shown as activity items in the user's profile.
• The snippets related activity is scoped to the projects the user contributes to.
• Comments on PersonalSnippets create events in the db but are not displayed as they don't belong to any project.
• Other types of events would need to be created as currently only comments are tracked.