Feat: Refactor is_public boolean to visibility enum on personal background

### Is your feature request related to a problem? Please describe.
The current is_public attribute limits users in their options in relation to their personal background.

### Describe the solution you'd like
as per discussed on [BIT Open Session](https://docs.google.com/document/d/1vMmLOC8txyOZdNfdFh6keB5rw7uEbL1F6dS_JdcnD0g/edit#heading=h.rh3yl7l0u155):
* is_public should be refactored from a boolean type and renamed `visibility` which provides options for users to opt their personal background to be:
* visible to all users
* visible to organizations only
* visible to BIT only

### Describe alternatives you've considered
NA

### Additional context
This issue will be addressed once #1092 is merged to `bit` branch