webview: Make it more flexible in terms of its size

So far, the minimum size of a GoaWebView was set to 500x400, which
meant that it couldn't be shrunk to fit smaller devices. This has now
been made more flexible by changing the current minimum to be the
default size, and using 300x200 as the new minimum.

https://gitlab.gnome.org/GNOME/gnome-online-accounts/merge_requests/26
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