Fix/mobile/login flow

[KamogeloMoeketse]

Description

Login on mobile now stores the token received upon a successful login in expo's encyrypted secure storage. When making requests in future, this token is retrieved and sent with the request as the auth header. This solves the problem on the login, logout issue for mobile.

Fixes #168

Type of change Fix

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules