New action `before_login_form`

[sebastian.pisula]

I can add new fields before default fields form. For example I can add social login :

​https://food52.com/users/sign_in?next_url=%2F

[sebastian.pisula]

I renew ticket

[swissspidy]

The patch would need some improved docblock description (see login_form hook for example) and an updated @since value. Also, should the hook name be login_form_before?

In general, I think it's time to finally make the login form more modular and testable.

[SergeyBiryukov]

Replying to swissspidy:

Also, should the hook name be login_form_before?

Given that it fires inside the form tag, I think login_form_top would be a better name, see ​comment_form_top for example.

If login_form_before was introduced, I'd expect it to fire before the form tag, see ​comment_form_before.

[SergeyBiryukov]

Related: #38079