Add GUI tests

[GuillaumeGomez]

crates.io has GUI tests using percy: https://percy.io/crates-io/crates.io/builds/11597533/changed/652575280

We had a lot of UI regressions so it'd be nice to have a GUI testsuite to make it easier to spot them (and to know where they happen more precisely).

Things to be tested:

• position of the footer (always at the bottom of the page)
• icons in the topbar depending on the width of the screen
• ...

[syphar]

duplicate of #996 ( I think)

[GuillaumeGomez]

Ah inedeed. Sorry about that.