Add multilingual support

[adrianq]

Closes #12

I have implemented multilingual support to the project. For now, I have added three languages (English, Spanish and Turkish) and three sampling terms. Name can be found in the registration form, Call in the tab label and TopUpContent is the dummy content for the top up tab.

We will have to migrate what we have implemented so far in order to start using this infrastructure: @MurhafSousli I think it would be good to do this as part of #23 @jordibc In your case, I added a bullet point in #21 @ifoche I think we can do this as part of #5

I also connected the i18n to the translation platform, POEditor. I will show you how it looks in our next FIRE meeting.

This PR should be merged after https://github.com/EyeSeeTea/FIRE-WiFiCalling/pull/18. For the sake of clarity, I am creating the PR targeting Murhaf branch (where I started from) but it should be merged against development when ready. Added you all guys as reviewers...

[adrianq]

@MurhafSousli as we discussed there are two bits missing here:

• Error terms
• Spanish and English terms for your developments. Add Spanish terms in English and I will take care of the translations

[MurhafSousli]

Following ngx-translate example here and here, let's use nested translations