Remove Gitter from codebase

We no longer use Gitter for development talk and similar. So we might
want to remove it?

This patch removes Gitter from README, help page and features page. And
replaces it in the help modal with POEditor, our translation platform.

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

README.md

@@ -3,7 +3,6 @@ CodiMD
 
 [![Standard - JavaScript Style Guide][standardjs-image]][standardjs-url]
 
-[![Join the chat at https://gitter.im/hackmdio/hackmd][gitter-image]][gitter-url]
 [![#CodiMD on matrix.org][matrix.org-image]][matrix.org-url]
 [![build status][travis-image]][travis-url]
 [![version][github-version-badge]][github-release-page]
@@ -364,8 +363,6 @@ See more at [http://operational-transformation.github.io/](http://operational-tr
 
 **License under AGPL.**
 
-[gitter-image]: https://badges.gitter.im/Join%20Chat.svg
-[gitter-url]: https://gitter.im/hackmdio/hackmd?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
 [matrix.org-image]: https://img.shields.io/badge/Matrix.org-%23CodiMD@matrix.org-green.svg
 [matrix.org-url]: https://riot.im/app/#/room/#codimd:matrix.org
 [travis-image]: https://travis-ci.org/hackmdio/codimd.svg?branch=master