Merge remote-tracking branch 'origin/contrib/15061-document-security-of-verification-extension' (Closes: #15061)

Merge remote-tracking branch 'cbrownstein/doc/15434-verification-extension-broken-in-3.6' into doc/15434-verification-extension-broken-in-3.6

replaceUrlPrefixWithRandomMirror() is not meant to be called twice on
the same elements and this leads to the URL of the mirror being mangled.

This commit introduces another class (.use-mirror-pool-on-retry) on
which replaceUrlPrefixWithRandomMirror() is called when the verification
fails. The download button with .use-mirror-pool gets hidden and
replaced by one with .use-mirror-pool-on-retry.

This not very elegant but probably the simplest solution.

This reverts commit 6d65d9ff.

Merge branch 'master' of https://git-tails.immerda.ch/tails into doc/15434-verification-extension-broken-in-3.6