implement login functionality for admin

[okieLoki]

Implement the loginAdmin function in the adminController to handle administrator login. Use json web tokens and bcryptjs.

[kj-1809]

assign

[okieLoki]

@kj-1809 we are currently limiting to one contribution per user

[gautam070205]

Please assign me this issue

[Sxhail]

Please assign this issue to me