Add a Remeber me checkbox to login that extend session time. (#3926)

Fixes #3897

"Remember me" is only available when logging in with password. I see
this as an "advanced" feature. The passwordless login flow is easy so
not a big hassle to do it once a day.

Organisation that wants to can set `SESSION_COOKIE_AGE` to a custom
value, e.g. set it to two weeks to reagin the old behaviour.

To disable "Remember me" set `SESSION_COOKIE_AGE` to the same value as
`SESSION_COOKIE_AGE_LONG`

This PR also:

* removes unused code is_public_site
* removes unused code register_extra_text
* removes all use of link-button* classes. Buttons are now only handled
by the button class.
* add a tiny border radius to all buttons (this is something we
implement on all elements in Hypha)