Translating starting message for subscribe button

[hmeza]

This adds translations for 2 messages that were left untranslated by gettext() Also allows translating the initial button message without need of copying webpush_button.html template to template/ project folder to override it.

[hmeza]

Hey @safwanrahman ! Noticed some activity on the repo. Just pinging you in case you accept these changes. If not, feedback welcome. Thanks!

[safwanrahman]

Looks good! Thanks @hmeza 🤩