Bugfix: Search icon would drift from input field

[Robadob]

Likewise, the search box appears to have a static size/position whilst visible (although the green magnifying glass does alot of walking about at widths >1200px).

This has been one of those frustrating bugs from the initial design that I haven't been able to pin down very well 😞

• justify-content-end tells the form to justify it's contents (the input field) to the right hand side
• position: relative makes position: absolute on children relative to the form, rather than the nav bar
• top/right: update the offsets to match

This also requires the margin value used in #113 to be changed, which I've temporarily done inside 4276b7a.

@zkamvar If you let me know what you think I can finish this off and update NEWS, it's stable for me in Chrome now best I can tell.

Although, it would probably be cleaner to try and rebuild the whole nav to properly use Bootstrap's layout classes which would hopefully negate the need for #113 (but I try to avoid playing with CSS, it's awful imo).