Offline js-fetch-sections DOM manipulations

When manipulating the DOM in order to add newly fetch data, the current implementation was causing some troubles in Chrome: Layout trashing was occuring between each append class into the select tag. While being inefficient, it was also causing visual glitches (See https://github.com/symphonycms/selectbox_link_field/issues/79).

The way to fix this is to "offline" those changes:

1. Detaching the select element from the DOM
2. Creating a detached list of all the option element
3. Append the detached list to the offline select element
4. Restore the select element to its original position
Removed unecessary translateZ hack

After much testing, this is not needed anymore for both latest Chrome and Safari.
In fact, there is now a penalty cost associated to it.
Fix indentation
New build
Merge pull request #2231 from DeuxHuitHuit/js-fetch-trashing

Offline js-fetch-sections DOM manipulations. Fixes symphonycms/selectbox_link_field#2231
Renamed `Gateway` class to `HTTPGateway`

As stated in #2229, the goal of this change is to make it more clear
what's this class is all about. I follow the principle that we already
have the EmailGateway (so prefixing "Gateway" with its related
"technology") and used UPPERCASE for HTTP since we already have the
`SMTPGateway` (uppercase) class name.
Replace every occurence of Gateway by HTTPGateway

According to previous commit, the Gateway class has been renamed to
HTTPGateway. This commit also covers the require_once calls.