feat: setup Gettext for internationalisation in emails

[spaenleh]

In this PR I set Gettext up.

• documentation was added to the Readme
• email templates support translations based on recipient's lang setting

Usage

To make some text in the app use translations imply wrap the text string with the gettext/2 macro:
Click here -> gettext("Click here") for example.
Interpolation is also possible: gettext("Hello %{name}", name: "John").
Pluralisation is supported with ngettext("One error", "%{count} errors", 3)

There is a simple mix task that can be used to update the translations: mix i18n

The translation template files are .pot files, they should not be touched. They are updated with mix gettext.extract.
The translation files are .po files, this is the files that are given to translators. They can be updates with the new, updated and removed keys with mix gettext.merge priv/gettext.