UX: Move emoji setting into 'Plugins' category

Rationale: currently, when I install  MathJax plugin I get two categories: Emoji and MathJax. Each has *only one item*. I think most of plugins have only one setting (enable/disable) which from UX point of view does not look good (visual cruft & waste of space).

The 'Plugin' category was discussed at meta: https://meta.discourse.org/t/create-a-settings-category-for-plugins/11018/7 but for some reason nobody introduced it in upstream and as a resut *each* plugin usually creates *its own category*. 

Why create 'Plugin' category here?
As Emoji plugin is included by default and has one setting it guarantees that every Discourse instance will have 'Plugins' category present by default.  Having that, other plugins could move their settings there.