* FEATURE: introduces minimum trust level for polls
This commit makes `poll_enabled` less misleading and introduces `poll_minimum_trust_level_to_create`. If poll are enabled they will always be cooked, and if you have the required trust level you can create polls. As a side effect, it also fixes a bug where rebaking a post created by staff member when `poll_enabled=false` would end up not cooking it.
It also adds more tests to ensure settings are respected.
* admins should be whitelisted
* checks for admin in post validation
* test for >= instead of == trust level
- add User.staff scope
- inject MessageBus into Ember views (so it can be used by the poll plugin)
- REFACTOR: use more accurate is_first_post? method instead of post_number == 1
- FEATURE: add support for JSON-typed custom fields
- FEATURE: allow plugins to add validation
- FEATURE: add post_custom_fields to PostSerializer
- FEATURE: allow plugins to whitelist post_custom_fields
- FIX: don't bump when post did not save successfully
- FEATURE: polls are supported in any post
- FEATURE: allow for multiple polls in the same post
- FEATURE: multiple choice polls
- FEATURE: rating polls
- FEATURE: new dialect allowing users to preview polls in the composer
