Customize auth providers buttons by overriding allauth's template

[gounux]

This PR customizes the display of the SSO providers buttons, in the login page.

Logic consists of overriding allauth's provider.html template, with style being configured in the QFIELDCLOUD_SSO_PROVIDER_STYLES settings variable (fill, stroke, text color + svg icon).

Since the QFieldCloud jazzmin UI is light, the light styles are being used at the moment.

Approach is to use a css file for non-dynamic style properties.