Modernize Conpherence access to user preferences

Summary:
Ref T4103. Conpherence is doing some weird stuff and has its own redudnant settings object.

• Get rid of ConpherenceSettings.
• Use getUserSetting() instead of loadPreferences().
• When applying transactions, add a new mechanism to efficiently prefill caches (this will still work anyway, but it's slower if we don't bulk-fetch).

Test Plan:

• Changed global Conpherence setting.
• Created a new Conpherence, saw setting set to global default.
• Changed local room setting.
• Submitted messages.
• Saw cache prefill for all particpiants in database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16025