feat(frontend): make language selection for translations more intuiti…

…ve (#461)

The PR modifies the translations section of the edit entry page:

* Changes "All languages" terminology to "Fallback translations" (fixes #458)
* Adds an info alert if "en" (English) or "xx" (Fallback translations) is not the main language for an entry (fixes #457)
/ Fixes the fact that the alert message about changing the display name of a language appears even if the language had no translations and we add the first one (fixes #459)
* Adds a "Show all existing translations" checkbox to see all the languages that currently have translations, with their translations Adds a possibility to "pin" languages to select them (so they stay in local storage and appear at the top for each entry), and a possibility to hide (unselect) these languages easily (with an icon next to their title)
* Modifies the selection of new languages: I removed the "number of languages shown" button that had to be clicked to add a language, and created a "Show another language" button at the bottom of the section. Also, the dialog is now an autocomplete instead of a select, and you just type the languages that you want to add and see languages that are not selected, instead of seeing all current languages and being able to remove them. The autocomplete with the options is also automatically focused when opening the dialog.
* Adds vite-plugin-svgr to easily import svg files in React

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Co-authored-by: Charles Perier <charlesperier@Charless-Laptop.local>