Bootstrap 4.x migration plan #1

ToDo:

1. Create new .svg spinner/trobbler (since Glypicons obsolette) (see: https://www.drupal.org/node/2208365 https://www.drupal.org/node/2775725) or use BS 4.1 (see:v4.2: Spinners! twbs/bootstrap#22960 twbs/bootstrap@86ca31d)
2. Do we need to migrate from grunt to npm scripts?
3. Consider to use gulp tasks or webpack in sass starterkit
4. Create task to compile _overrides.scss upon module commit
5. Consider to remove jsDelivr an use maxcdn.bootstrapcdn.com as default CDN provider?
6. Consider to remove Bootswatch (there isn't API support for BS4 at this time)?
7. Remove panels (obsolette) support from starterkits (code+templates) and use cards instead
8. Migrate from Tether.js (obsolette) to Popper.js for tooltips (in composer.json)
9. Check pagination markup template
10. Check that theme suggestions works correctly for cards and other elements after Add _lite suffix #2 fix
11. Check tabs markup template
12. Change default variables (main theme & starterkits) for conformance with BS4
13. Revamp _overrides.scss (main theme & starterkits) for conformance with BS4
14. Remove wells (obsolette) support (code+templates) and make decision to use cards instead
15. Revamp dropdowns (single & inside navbars)
16. Remove Glypicons font (obsolette)
17. Revamp navbar markup template Revamp navbar markup template #10
18. Update logo.svg to v.4 logo (base theme & subthemes)

Fixed:

1. Remove LESS starterkit (obsolette) #ca67004
2. Convert col-sm-* to col-md-* in twig templates Add _lite suffix #2
3. Change breakpoints size in bootstrap.breakpoints.yml
4. Add theme suffix "_lite" (prepare to commit to 4.x-dev branch of bootstrap_lite theme) Add _lite suffix #2
5. Create new branch 8.x-4.x and delete master
6. Update favicon.ico (16x16 & 32x32) Update favicon.ico #4
7. Remove all deprecated code Remove all deprecated code #3
8. Update screenshot.png
9. Convert img-responsive to img-fluid Convert img-responsive to img-fluid #5
10. Update image shape classes Update image shape classes #6
11. Check tables markup template Revamp table classes #7
12. Revamp table classes Revamp table classes #7
13. Remove panels (obsolette) support from base theme (code+templates) and use cards instead Remove panels (obsolette) support from base theme (code+templates) and use cards instead #8
14. Remove affix jQuery plugin (obsolette) Remove affix jQuery plugin #9