Use styled-components in KeyboardShortcutModal

[lindapaiste]

Progress on #1760

Changes:

• Migrate styles from client/styles/components/_keyboard-shortcuts.scss into individual styled components in the client/modules/IDE/components/KeyboardShortcutModal.jsx file (Container, Command, etc.).
• Moved around some paddings and margins to minimize complex selectors.
• Moved two untranslated strings into the translations/locales/en-US/translations.json file.
• Created a Shortcut component to be used for each item to minimize repetitive markup.
• Looks identical to before, at least for me on Windows Chrome.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[PiyushChandra17]

Nicely Done 👍