Remove n+1 queries from conference index and conference show pages

[andrewmile]

This PR removes n+1 queries with the following solutions:

• The count of open issues is now being loaded on the conferences index and show pages
• Submissions are now loaded on the conferences index page
• Each talk's current revision is now loaded on the conferences index and show pages
• The talk of the current revision is set as a relation on the conference show page

This results in the following query count reduction when testing locally:

Page	Before	After
Conference list	80	1
Conference show	2	2
Conference list (auth)	163	6
Conference show	125	7

[mattstauffer]

Looks good to me! Great work, and I really appreciate extracting the currentVersion; it's somethign I know to do today, thanks to some wonderful database teachers and new Eloquent features, but could only wish for back when I wrote this. (I think I wrote this? And the current() is my fault? lol)

I believe this work was co-authored with @omarrida--since he's not marked as a co-author (that's fine, i forget often too), just wanted to credit him in this PR for future knowledge.