StandardFooter requires a global level vue-i18n instance

[davinotdavid]

Describe the bug
We are using $t() and t() interexchangeably when dealing with i18n. However, whenever we use $t(), it tries to find a global instance of the vue-i18n lib to be able to retrieve the translations.

This is a problem because for consumers of services-ui that don't have vue-i18n setup with the required keys, it would be impossible to override its default text.

Other components might also be affected!

To Reproduce
Steps to reproduce the behavior:

• Make a Vue app without vue-i18n setup (or use https://github.com/thunderbird/tbpro-add-on)
• Install services-ui and try to use StandardFooter
• See that the "Thunderbird is part of MZLA Technologies Corporation..." text doesn't show up

Expected behavior
We shouldn't require the keys to be defined on a per-project basis for these texts