add event page

[dedemenezes]

This PR implements an events#show page as discussed.

Close #7

The page added has the same structure as the speakers#show.
What I'm adding:

1. System test for the events#show, ensuring you can see the event name and talks for the event.
2. Event show page, displaying all talks to the specific event using pagination.
3. Link to events#show on the talks/card partial.

What I'm missing:

1. The back link on talks#show always take the user back to a speakers#show, at the moment it redirect to the first speaker for each talk. I see there is a PR trying to fix it (Fix back navigation #26), so I was not sure if I should do it her.
2. The about information for the event is actually coming from the organisation as mentioned in Start cleaning up the test suite #37. This means that the events#edit is buggy and that's why I kept the edit button as a dead link. Should I remove the button while there is no edit action?
3. The list of speakers is quite big. Should be fixed by Show a few speakers #28

Here is a printscreen how it looks like atm 👇

[adrienpoly]

Thanks this looks very cool and thanks for identifying the various blockers from the ongoing works. Will test it right away