Add feature: is-expanded to navbar-item #1255

[doatech]

This is an improvement.

Added .is-expanded modifier for navbar-item suggested in #1255

Proposed solution

This is a PR for issue #1255 using basic solution suggested by jgthms.

Testing Done

Done locally using Laravel 5.5's default mix npm run dev & npm run prod

[jgthms]

Just need to fix the changelog conflict.

[doatech]

@jgthms fixed!

[amir20]

@jgthms I came to this issue and tried using .is-expanded on .navbar-item. However, it wasn't working. After some debugging, I realized .navbar-start doesn't have a size so the flex box was never growing. Do you know what needs to happen to .navbar-start for this to work?

[doatech]

@amir20 I suggest adding flex-grow: 1; flex-shrink: 1; to your .navbar-start, but that will make the .navbar-start expanded too. Or maybe use just .navbar-item-s without .navbar-start.