Re-add blank 'goa-account' icons

The non-scalable versions of these icons were removed in b8a6f2a8, but no scalable version was added. When no provider-specific icon is available (as is the case for 'goa-account-exchange'), GIcon tries to fall back to 'goa-account', then 'goa'. Since neither of those icons exists either, applications show the 'missing icon' icon.

Restore the pre-3.31.90 behaviour of showing a blank space where the Exchange icon should be in Settings by re-adding the blank 'goa-account' icon.

Partially solves #8 (closed). I suggest this could be shipped on the 3.34 branch.

Before:

After:

Not perfect but an improvement, and we lived with this for 6 years…