added modals & changed zindex's

- added Modals Demo Page
- added Link to Modals in any other Demo Page
- added Modal Color Variation fixes
- updated zindex values `header` from 1000 to 900 `sidebar` from 1100 to 950 (below bootstrap 4 defaults to work with dropdown, modal, popover & tooltip)
- removed tooltip zindex overide

Some little V3-dev changes/fixes

- bootstrap 4.1.2 to 4.3.1
- chart.js 2.7.2 to 2.8.0
- jquery 3.3.1 to 3.4.1
- popper.js 1.14.3 to 1.15.0
- browser-sync 2.24.5 to 2.26.5
- clean-css-cli 4.1.11 to 4.3.0
- eslint-plugin-compat 2.5.0 to 2.7.0
- node-sass 4.9.2 to 4.12.0
- nodemon 1.18.2 to 1.19.0
- npm-run-all 4.1.3 to 4.1.5
- uglify-js 3.4.4 to 3.5.13

Fix wrong label column number

It causes label text to wrap to the next line

slimscroll on V3 dev  #2082

* fix(scss): same text-color as background for nav-item:hover dropdown

https://github.com/almasaeed2010/AdminLTE/issues/1893

* fix(scss): same text-color as background for nav-item:hover dropdown

https://github.com/almasaeed2010/AdminLTE/issues/1893

https://github.com/almasaeed2010/AdminLTE/issues/1910