Updated Google login button (bitfireAT/davx5#332)

* Updated Google login button

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Moved Google G logo file

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Changed background color

Signed-off-by: Arnau Mora <arnyminerz@proton.me>

* Amend warning sign [skip CI]

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Signed-off-by: Arnau Mora <arnyminerz@proton.me>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>