Admin: use select2

[markets]

WHAT?

Basically this 👉🏼 #614 (comment)

EXTRA

• 💎 Add https://github.com/Shopify/bootsnap to speed up boot time (default gem since Rails 5.2)
• ✂️ to-categories-dropdown.scss => we already have Bootstrap helpers for that
• 🐛 fix some i18n keys (implicit scopes doesn't work well with partials and helpers, they are usually called in different places)
• 🔧 code style refactor in Movement#other_side method + test

[markets]

@sseerrggii you got it! 💪🏼

[sseerrggii]

cool 👍

Tested ✔️