Allow events to specify a URL for ticketing

[itsrachelfish]

It would be useful if the volunteer database tracked a URL that volunteers can use to buy tickets for the event.

Myschievia requested that we update this email notification:

Currently the template for this email simply uses the event name which works for things like Apogaea and DeCOmP but it would be much more useful to include an actual ticket URL.

volunteers/laravel/resources/views/emails/user-shift-confirmation.blade.php

Lines 1 to 7 in 41e132c

	<h1>You're signed up for a {{ $shift_name }} shift at {{ $event_name }}!</h1>
	<p>
	Your shift will take place on {{ $start_date}} between {{ $start_time }} and {{ $end_time }}.
	</p>
	<p>
	For volunteer ticketing information, please visit the {{ $event_name }} ticketing webpage.
	</p>

A ticket URL would be useful to display in this email as well as on the volunteer shift page itself.