Fixing bug - miscellaneous minor fixes (found during tests)

[topi-chan]

Issue
There are some bugs which should be fixed.

Tasks

• 1/ after hr user tries to add an employee to his company - if the employee is already registered into another company, the hr user can see "dodano pracownika" banner, but nothing happens (he should see a different banner) - @topi-chan

• 2/ serializers may be reviewed - if there aren't unnecessary fields (i.a. for security reasons); @adam-sas-on @topi-chan

• 3/ we can add url '/login' for intuitive login sub-site (currently this url is not registered); @topi-chan

• 4/ email template is too bright / contrast and should be redesigned @annapakula & @Michal-Roszyk

• 5/ page tittles are in English (for example - "employee dashboard"); - to be checked

• 6/ after a form is saved by the user (an employee) or sent to hr manager he is not redirected into neither list sub-site, nor main page, which may be unintuitive for the user.

First issues were identified by myself, last one by Michał.
...to be continued?

For checks: deleting already send form is not working:
django.db.models.deletion.ProtectedError: ("Cannot delete some instances of model 'Package' because they are referenced through protected foreign keys: 'PackagesUsers.package'.", <itertools.chain object at 0x7fe88baa4dc0>)
[20/Mar/2021 00:57:23] "DELETE /api/package/8/ HTTP/1.1" 500 16871