cherry-pick maint-14.0 torbrowser_legacy_* related patches to maint-14.5

We've various patches in maint-14.0 we will need to carry in maint-14.5 to keep going with legacy 13.5 releases. From a comment:

@pierov posted:

Looks like we might need to cherry-pick the 13.5-related changes to maint-14.5 (the ones to consume var/torbrowser_legacy_version and var/torbrowser_legacy_platform_version).

They might be 0a476854, b585a1a9, and possibly more.