🆕 🔧 Issue templates: Add Medium/Large Event template

This commit adds a new GitLab issue template specifically for Medium Events and Large Events, as documented in the Fedora Mindshare Committee event policy docs. Since the requirements for Medium and Large Events are very similar, I used a single issue template to include requests for both of these kinds of events.

The main difference from the Small Events issue template is that the requested support section is longer and more detailed. There is room for someone to specify travel and accommodation needs, as well as any other costs for an event to be successful.

Once this template is added and the Small Events template is updated, the "Event Requests" repo can be repurposed to only focus on receipts and documentation, instead of asking a submitter to re-explain their entire event in another ticket.