Allow the side entrace login page to be used as fallback for Moodle setups with external IdPs.

[krostas1983]

Is your feature request related to a problem? Please describe.
Logging in with a manual site admin account on a site where alternateloginurl is set may pose a challenge. Boost Union provides a feature for this - /theme/boost_union/locallogin.php - but this is currently hidden behind an option loginlocalloginenable that is related to the default login page (which may not be in use at all when alternateloginurl is set).

Describe the solution you'd like

• As a site admin, I would like to enable / disable the fallback local login page independently of other settings
• As a site admin, I would like to enable a reliable way for users / admins with manual accounts to access the site

Describe alternatives you've considered
As discussed in #776 , the original idea was to extend the logic in locallogin.php to include cases where alternateloginurl is set. This is somewhat opaque and feels more like an easter egg than a feature.
Another alternative currently in use is a side entrance "clone" of the /login folder with some adjustments which is redundant and needlessly bloating up our installations - especially when Boost Union already provides a suitable feature.

[wiebkemueller-hsh]

#777 I would like to link this, as we just discussed about this with Alex today.