URL should not be required on login when it is predefined on global storage

[liorb-canva]

Context:
For self hosteed coder server, a private url needed to be provided in login screen
This url is not alweys intuative and easy to remember

Accepted behaviour
If the URL is defined in global config or in the CODER_URL environment variable, do not show the user a prompt to fill the url, and redirect to the pre-defined url to login.

Current Behaviour
The login function, ignored the url stored in Storage class

[matifali]

While doing this, make sure it can also handle multiple coder deployments gracefully.
Related: #173

[code-asher]

I think we might still want to show the login prompt but just prefill it, otherwise what if you want to connect to a different deployment but it keeps auto-logging you into a different one?

[matifali]

The best would be to add the ability to list recently used URLs and prefilled the last connected one.