Refactor backend controllers to be typed as RequestHandler

[LoneRifle]

Some of the backend controllers are typed incorrectly. express middleware functions should be typed as RequestHandler (can import from ‘express’) and one should always supply generic type arguments to specify the shape of the route params, response body, request body and query params.

Rationale

If you go to getSingleAgency and try to access req.body.nonsense, req.query.nonsense or req.params.nonsense, you can see that they are all either any or string, even though we know that these are undefined. this is because there are no generic type arguments to Request and Response, so everything defaults to any or string.

Example

https://github.com/opengovsg/FormSG/blob/f4db38bde03c59ec0b8856bfed08f0f31bdcb538/src/app/modules/submission/email-submission/email-submission.controller.ts#L34-L38