Offline js-fetch-sections DOM manipulations

This is RE: https://github.com/symphonycms/selectbox_link_field/issues/79

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.

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

This PR also contains some really minor css changes and a new asset build.

I would also be very glad if this PR would fit in 2.5.2 release as I have a lot of site plagued with this...