iss#492 Replaced the username label with epost label for login form

[lenaar]

there is no actually any translation for login.form.email (field label) and login.form.email.validation (error message), there is only for username.
The solution: I can reuse user.form.email it has right translation, but then for error message will be 'you need to enter a valid email' instead of 'Epost missing'

Closes #492

[buren]

Thank you @lenaar!

I see one risk with this (I might be overly paranoid here..) and that is if we want to update the copy in the user registration form to something that doesn't really make sense in the login form.

tl;dr Add a mention to the caveat mentioned ☝️ then we can merge this :)

---

I see 3 ways we can do this.

1. Go with this solution (but perhaps add a comment about the caveat mentioner ☝️)

2. Refactor the a bit in the translation files to include some "common/generic translations" that can be reused in multiple places (which would no doubt require some work since we don't want to re-translate a lot of stuff so this would require some coordination)

3. Only change:
   https://github.com/justarrived/just-match-frontend/blob/master/config/translations/en.json#L61
   https://github.com/justarrived/just-match-frontend/blob/master/config/translations/en.json#L63
   to

   # Before
   "login.form.username": "Username",
   ...
   "login.form.username.validation": "Username missing",
   
   # After
   
   "login.form.email": "Email",
   ...
   "login.form.email.validation": "You need to enter a valid email",
   

   and then do the same for the other locales (sv, ar, fa_AF, ps, ku..)

Actually I think that option 1 is the best, but thought it would be a good idea to scribble down these thoughts somewhere ;)