Add i18n support

[oliverviljamaa]

Closes #39.

Context

For the Estonian deployment, we'll need to internationalise and localise the app to the Estonian language 🇪🇪.

Currently, the copy is hardcoded in the components in English 🇬🇧.

Changes

• retranslate translation library is added
• English messages are moved from inside the components to the messages file (src/i18n/en.json)

Considerations

Left to do:

• adding Estonian messages (Estonian messages #48)
• formatting dates depending on the language (Estonian date formatting when using the Estonian language #49)
• logic for determining which language to use (Logic for determining which language to use based on the deployment #50)
• adding i18n support for messages coming from the back-end, e.g. test results (optional) (Back-end i18n and l10n backend#103)