conferences that use disabled plugins are not listed

fixes CNVS-10839

test plan
- enable wimba and adobe connect conference plugins
- make a conference
- ensure it is visible on the conference index page
- disable the plugin for the conference
- ensure it is no longer visible on the conference index page

Change-Id: Idc26d306f683fade5bf6d8619dfe9325047bb116
Reviewed-on: https://gerrit.instructure.com/29487
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>