Remove `bootstrap3` package from project and use native forms

[karbassi]

Remove bootstrap3 package from project and use native django forms.

Since we're not using bootstrap for this project anymore, this package is not that useful.

For example, home_mentor.html has the following:

https://github.com/CoderDojoChi/coderdojochi/blob/d81539329dba4c2bac334759263c41d61f444de5/accounts/templates/account/home_mentor.html#L68-L76

We can use native forms: https://docs.djangoproject.com/en/2.1/topics/forms/

Moving the forms from the accounts/views.py to the correct spot at accounts/forms.py

https://github.com/CoderDojoChi/coderdojochi/blob/d81539329dba4c2bac334759263c41d61f444de5/accounts/views.py#L46-L139