[User-Sign-Up] user can register for a particular role

A user should be able to register as a teacher or a trainer.

A teacher is one who will be involved in directly impacting members of the community he/she is (will be) assigned to.

The job of the teacher is to inculcate knowledge, and educate members of his/her assigned community.

# Request data
- first name: must be between 2 and 30 characters long
- last name: must be between 2 and 30 characters long
- email
- phone number
- password: must include at least a number; must be between 6 and 30 characters long; must include at least a special character in the range of full stop (.), asterisk (*), ampersand (&), caret (^), exclamation (!), dash(-), and underscore(_)
- country of residence
- state/province residence
- city/town of residence
- fluent languages
- role: must be either teacher or trainer

# Implementations
- Make use of the `express-validator` middleware
- Encrypt user password with `bcrypt`
- Send an activation message and link to the user email
- The activation link must include certain crypted info about the user

# Response data
- Send user an informative success message to activate his/her account in the provided email