fix: fix-links behavior due to a problem with external links substitution

[luca-bellenghi]

Problem is that if we change testurl.it with correcturl.it on a link ct
the subs it's always changing with a site relative domain.

eg.
testurl.it/testA will be always changed with /${portal_url}/resolveuid/1234567890
with 1234567890 the UID of testA.

if correcturl.it it's a different site this behavior will brake the link

[arsenico13]

I totally agree with this PR.

This change avoid the mentioned problem and let us use this utility-view for a lot a use cases (like the one in which we discovered this bug).

[coveralls]

Pull Request Test Coverage Report for Build 6481104628

• 1 of 8 (12.5%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 62.429%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/redturtle/volto/browser/fix_links.py	1	8	12.5%

Totals
Change from base Build 6469968137:	-0.3%
Covered Lines:	766
Relevant Lines:	1227

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💛 - Coveralls