Migrate to Bootstrap 4

[php-coder]

Related resources:

• 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 (Use icons from FontAwesome #217)
• might require removal of bootstrap-languages as it depends on Bootstrap3 (Get rid of usrz/bootstrap-languages dependency #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
  • <meta viewport>: append shrink-to-fit=no
  • <link href>: use new version
  • <script src>: use new version
  • 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 twbs/bootlint#304 and twbs/bootlint#431 (usage of bootlint has been removed in #1669)