Base font styling on the computed style of the editor element

Previously, font styling was always explicitly assigned via the config.
This commit is the first step in basing the font styling of the editor
on the styles assigned via CSS. This will allow the editor's
font-family, font-size, and line-height to be assigned via CSS just like
they are for any other element, which will make it easier to style mini
editors.

We still need to switch the font settings to adjust a global stylesheet
rather than updating inline styles on each editor individually.
Only handle stylesheet changes after initial measurement
Handle editor font config options with a global stylesheet

Previously, each editor observed font-related config values on its own
and applied inline styles to honor them. This made it difficult to style
the editor like a normal element with CSS.

Moving this to a global stylesheet that targets editors via the .editor
selector means that the font size setting can be overridden in specific
contexts, such as when using mini editors.
Measure DOM in EditorComponent when a stylesheet is updated
Merge pull request #3106 from atom/ns-react-css-font-styling

Apply React editor font styles via CSS instead of inline styles