Improve frontend accessibility

...nfig/edit-custom-config/edit-custom-config.component.html

...t-video-settings/my-account-video-settings.component.html

...t-video-settings/my-account-video-settings.component.scss

...ccount/my-account-videos/my-account-videos.component.html

...ccount/my-account-videos/my-account-videos.component.scss

client/src/app/shared/forms/peertube-checkbox.component.html

client/src/app/shared/forms/peertube-checkbox.component.scss

client/src/app/shared/forms/peertube-checkbox.component.ts

client/src/app/shared/misc/help.component.html

client/src/app/shared/misc/help.component.ts

client/src/app/shared/shared.module.ts

client/src/app/shared/video-caption/video-caption.service.ts

client/src/app/shared/video/video-miniature.component.html

...c/app/videos/+video-edit/shared/video-edit.component.html

...c/app/videos/+video-edit/shared/video-edit.component.scss

...src/app/videos/+video-edit/shared/video-edit.component.ts

client/src/app/videos/+video-edit/video-update.component.ts

...nt/src/app/videos/+video-watch/video-watch.component.html

client/src/app/videos/+video-watch/video-watch.component.ts

client/src/assets/player/peertube-videojs-plugin.ts