Add pagination for admin views

[KattyaCuevas]

Issue: #64

Description
Add pagy gem for pagination and use the pagination helper in the events#index view

Requirements from the issue

• The GET admin/events endpoint accepts a page parameter
• The endpoint uses this parameter to determine which events to display on the page (i.e., if page is 1, display the first 25 events; if page is 2, display events 26-50, etc.)
• UI is added to the view, allowing the user to navigate between pages if more than one results page exists.
• If only one page of results exists, the UI does not allow the user to navigate between pages

UI:

[KattyaCuevas]

@luciagirasoles this PR is ready for review 💯

[luciagirasoles]

Thank you Kattya