[web][native] Trimming whitespaces from username at login screen

Summary:
Solving this issue: https://linear.app/comm/issue/ENG-2041/[web]-[native]-bug-whitespaces-arent-trimmed-from-right-side-of

Added onBlur event listener to username field so whenever it goes out of focus, it is trimmed.
It goes out of focus whenever we swap fields or click submit button, so it works.

On native we simply trim whitespaces whenever user types them as blur events aren't handled in a same way (input field doesn't go out of focus when we click submit button, so there's no good place for trimming it)

Mobile:

Web:

Test Plan:
Build application.
Tested scenarios manually.

Reviewers: tomek, ashoat, atul, kamil

Reviewed By: tomek, ashoat

Subscribers: abosh, kamil, atul, ashoat, tomek

Differential Revision: https://phab.comm.dev/D5365