Bugfix/bootstrap 4 update

[AlexNetcare]

So I tried to remove Bootstrap 3 from the build.gradle and replace it with Bootstrap 4 as I questioned here #1511.
I replaced the .less files with the newer .scss files. And I hope I did it correctly.
I looked through some pages and fixed the layout were applicable. There are still quite the differences, like some colors or line heights. But in general it feels very similiar. Having some additional thoughts on improvements and what should be adjusted, are appreciated.

[fhoeben]

@tcnh I believe you looked at the theming and bootstrap styling before. Can you take a look at these changes?

[AlexNetcare]

@tcnh Is there any progress on this or did I make too many errors that it is just too much to read?

[tcnh]

Hi @AlexNetcare, I haven't had an opportunity to look at this as I've been away for a while this summer. I will make some time next monday!

[AlexNetcare]

Hi @tcnh, did you have a chance to take a look at this?

[tcnh]

I started a while ago, but I can't find the time for a proper review. Sorry for that.
It seems everything works as expected, but I do see a quite some small misalignments caused by margins and paddings that feel a little off.
I find it hard to have an opinion on this, because I haven't seen the original fitnesse bootstrap theme in years as I've been using the bootstrap-plus theme that includes bootstrap 4 for a long time.

[AlexNetcare]

Some values changed from px to .rem which led to some of the misalignments. Also some colors changed even though the used css classes didn't change. I tried to avoid using custom css where possible, so I sticked with what it is right now.