feat: I18n (#1653)

* feat: i18n

* Changing language single source of truth from TEAM to USER

* Changes according to @tommoor comments on PR

* Changed package.json for build:i18n and translation label

* Finished 1st MVP of i18n for outline

* new translation labels & Portuguese from Portugal translation

* Fixes from PR request

* Described language dropdown as an experimental feature

* Set keySeparator to false in order to cowork with html keys

* Added useTranslation to Breadcrumb

* Repositioned <strong> element

* Removed extra space from TemplatesMenu

* Fortified the test suite for i18n

* Fixed trans component problematic

* Check if selected language is available

* Update yarn.lock

* Removed unused Trans

* Removing debug variable from i18n init

* Removed debug variable

* test: update snapshots

* flow: Remove decorator usage to get proper flow typing
It's a shame, but hopefully we'll move to Typescript in the next 6 months and we can forget this whole Flow mistake ever happened

* translate: Drafts

* More translatable strings

* Mo translation strings

* translation: Search

* async translations loading

* cache translations in client

* Revert "cache translations in client"

This reverts commit 08fb61ce36384ff90a704faffe4761eccfb76da1.

* Revert localStorage cache for cache headers

* Update Crowdin configuration file

* Moved translation files to locales folder and fixed english text

* Added CONTRIBUTING File for CrowdIn

* chore: Move translations again to please CrowdIn

* fix: loading paths
chore: Add strings for editor

* fix: Improve validation on documents.import endpoint

* test: mock bull

* fix: Unknown mimetype should fallback to Markdown parsing if markdown extension (#1678)

* closes #1675

* Update CONTRIBUTING

* chore: Add link to translation portal from app UI

* refactor: Centralize language config

* fix: Ensure creation of i18n directory in build

* feat: Add language prompt

* chore: Improve contributing guidelines, add link from README

* chore: Normalize tab header casing

* chore: More string externalization

* fix: Language prompt in dark mode

Co-authored-by: André Glatzl <andreglatzl@gmail.com>

README.md

@@ -164,6 +164,7 @@ However, before working on a pull request please let the core team know by creat
 
 If you’re looking for ways to get started, here's a list of ways to help us improve Outline:
 
+* [Translation](TRANSLATION.md) into other languages
 * Issues with [`good first issue`](https://github.com/outline/outline/labels/good%20first%20issue) label
 * Performance improvements, both on server and frontend
 * Developer happiness and documentation