Upgrading Bootstrap v3 to v4 and others things

[RomainOdeval]

Honestly, i don't think you will merge this but anyway, this is the all story. With a streamer team, we decided a year ago to use BatFlat for its simplicity. But we needed to have some upgrades on mobile display and FontAwesome. So we worked a lot and well ... this is it ! A monster PR ^^"

I don't think you can use it like this, specially because we don't clean completly the basic templates but Admin is fully OK (visually and technically) and the basic templates are technically fonctional, jsute not completely "beautiful".

thank you for you great simple CMS and i hope you will use this PR.

• Update Bootstrap 3.x to Bootstrap 4.x
• Update JQuery 2.x to JQuery 3.x
• Update FontAwesome 4.x to FontAwesome 5.x
• Modify Core to use/display FontAwesome classes (regular, solid, brand, ...) on Admin templates
• Fix Core to not display PHP error if somebody try to connecto to admin with a non-existing login
• Remove all lightbox gallery plugin to add http://ashleydw.github.io/lightbox/ instead
• Adapt and clean JS, CSS and Templates to use those 3 previous updates
• Fix some typos and translations

[michu2k]

Hi @RomainOdeval Thanks for your pull request! I don't think this pr will be merged, but it has some nice changes that can be used in future updates. Good Work!

[RomainOdeval]

Cool ! Honestly, i understand : a big PR with only one or two commits and change a lot of things ... But yeah, i think it can be useful to "pick ideas" to improve BatFlat core.

With my association, we will keep it and use it for our future website (with a private fork) because we want up-to-date tools but i hope a day you will use some ideas.