Migrate to Bootstrap 4

[php-coder]

Related resources:

• [x] https://getbootstrap.com/docs/4.3/migration/
• [ ] https://designmodo.com/migrate-bootstrap-4/
  • [ ] rename: input-sm -> form-control-sm
  • [ ] drop form-group-sm
  • [ ] panels, thumbnails and wells were removed and replaced with cards
• [ ] http://upgrade-bootstrap.bootply.com/
• [ ] https://github.com/creativetimofficial/bootstrap-converter
• [ ] https://designmodo.com/bootstrap-4-grid-system/
• [ ] https://designmodo.com/bootstrap-4-flexbox/
• [ ] https://designmodo.com/bootstrap-css-part1/
• [ ] https://designmodo.com/bootstrap-css-part2/
• [ ] https://webref.ru/layout/bootstrap4
• [ ] https://www.youtube.com/watch?v=QAgrHLtG1Yk&list=PL4cUxeGkcC9jE_cGvLLC60C_PeF_24pvv
• [ ] https://medium.com/@lukaszholeczek/migration-from-bootstrap-3-to-bootstrap-4-4ec63a1dd8b1
• [ ] https://www.beyondjava.net/migrating-bootstrap-4
• [ ] might require using of FontAwesome (#217)
• [ ] might require removal of bootstrap-languages as it depends on Bootstrap3 (#246)

TODO:

• [ ] series/info.html
  • [ ] glyphicon glyphicon-ok
  • [ ] glyphicon glyphicon-remove
• [ ] series/partial/comment.html
• [ ] series/add.html
  • [ ] glyphicon glyphicon-chevron-down
• [ ] series/import/info.html
• [ ] series/import/list.html
• [ ] series/import/request.html
• [ ] series/search_result.html
• [ ] collection/info.html
• [ ] collection/estimation.html
• [ ] category/info.html
• [ ] category/list.html
• [ ] category/add.html
• [ ] participant/add.html
• [ ] site/index.html
• [ ] site/events.html
• [ ] account/auth.html
• [ ] account/register.html
• [ ] account/activate.html
• [ ] error/status-code.html
  • [ ] update commented out glyphicons
    • [ ] glyphicon glyphicon-user
    • [ ] glyphicon glyphicon-share
• [ ] country/info.html
• [ ] country/list.html
• [ ] country/add.html
• [ ] nginx/503.en.html
  • [x] <meta viewport>: append shrink-to-fit=no
  • [x] <link href>: use new version
  • [x] <script src>: use new version
  • [x] col-sm-12 => col-md-12
  • [ ] glyphicon glyphicon-envelope => ???
• [ ] nginx/503.ru.html
• [ ] update & check CSP rules
• [ ] update server rendering (update webjar, modify constants, etc)

~~Ensure that bootlint is ready (or stop using it): see https://github.com/twbs/bootlint/issues/304 and https://github.com/twbs/bootlint/issues/431~~ (usage of bootlint has been removed in #1669)