[UI update] Lock left and right panels to the sides of the screen so they no longer render "over" the canvas.

RE: #4

1. Why it was an issue to have sidebars on top of canvas ? It was/is still possible to drag and zoom.
It is (and will remain) possible to pan/drag/zoom even with the new "locked" sidebars. The change was to free up more space on smaller screens. We wanted to make better use of the limited space the experience builder UI should take up on the page by reducing the large gaps around the panels. This change also addresses some usability issues with the browser scroll bar being on the right-hand side of the page, right of/behind the right panel.

2. Will it be still possible to see desktop and mobile at same time ? It's quite handy to have both at the same time.
This is unlikely to still be possible however the current UX designs allow for a quick switch between different sizes. Once we show only one viewport size at a time I would like to (at some point) make the iFrame manually resizable. The changes are for 2 main reasons
a) When you have two views side by side, quite often the content doesn't line up. Once the content on a site is fairly long, the mobile view tends to be significantly longer than the desktop due to being much narrower and as a result you can't easily view the same piece of content in both viewports at the same time anyway
b) performance of rendering the site multiple times (with our overlay that shows all the borders etc on hover) was starting to be a limitation.

3. Any plan to make right sidebar (maybe left too) resizable ? I can image a prop with ckeditor could need lots of space for example
Yes! See point 9 in the summary of #3521714: [Meta] Application "Shell" design refactor and have a sneak peak at some of the designs (still subject to change)!

Further to the above, this is just one step towards a fairly big redesign of the overall layout that is more future proof because it frees up space and sets in place a number of UI patterns (such as the new side menu) that will allow us to more easily add more features over time.