Make react the default editor

This will make react the default editor.

All issues related to the react editor: https://github.com/atom/atom/issues?labels=react-editor-bug

## Todo 

These are the blockers to releasing the react editor as the default

* [ ] Do a pass on React updates and see if we can boost cursor and text insertion performance

## Completed Tasks
* [x] Show code folding markers in gutter #1984 #2464
* [x] Shift + Backspace renders deleted characters #2159 
* [x] Lines sometimes duplicated or not removed #2429
* [x] Make autocomplete-plus work with the react editor #2318
* [x] Fix broken context menus #2442
* [x] Missing API on ReactEditorView #2428 (PR #2497)
* [x] React editor cannot open this file  bug react-editor #2392 
* [x] Closing find occasionally causes buffer rendering issues #2237 
* [x] Get the build green with react turned on in specs
* [x] Fix selection problems #2459
* [x] Fix problems with spellcheck #2460
* [x] Fix issues with find and replace atom/find-and-replace#232
* [x] Scroll speed is too fast
* [x] Selections become hidden behind tool-panels #2578 
* [x] Add back IME composition events 184068dc55d2cc2f3aaa9e43c7aa9aa54946b53d
* [x] Add highlight decorations #2605 
* [x] Gutter styles don't extend to the bottom of the editor #2596 
* [x] With React editor turned on line numbers are always on #2707 
* [x] Decorations not updated when markers move #2705 #2727 
* [x] ctrl+click to add a cursor doesn't work on linux #2105 
* [x] ReactEditorView has no method resetDisplay #2674 
* [x] Mouse selection seems to be off-by-one in React Editor #2668
* [x] Editor scroll width is miscalculated when longest line has variable-width characters #2666 
* [x] Cursor line highlights don't go full width and show with selections #2701 #2757  
* [x] Create blog post about the React Editor change atom/blog.atom.io#48
* [x] Soft wrap issues #2842 #2844 
* [x] Character widths aren't invalidated when stylesheets are changed #2845 
* [x] Not autoscrolling when cursor moves due to undo #2815
* [x] Indent guide is wrong #2367
* [x] Update the decoration API #2819
* [x] Characters are measured as 0 width when editor is hidden during theme change #2856 
* [x] Accent circonflexe removes previous char #2716 
* [x] End-of-line invisibles not visible on empty lines with indent guides in React editor #2857
* [x] Scrolling is sluggish when there a large numbers of folds #2848 

## Fixes these issues

Fixes #2443
Fixes #1850
Fixes #1790 
Fixes #1711
Fixes #2778
Fixes #2764
Fixed #2710 
Fixed #2785
Fixed atom/find-and-replace#245
Fixes #2375