Customize: Allow themes to be installed when SFTP credentials required

[celloexpressions]

Split out from #42184.

Next steps:

• Revert [41997]
• Use 42184.2.diff
• Fix the two known bugs: incorrect labels after cancelling the modal, and fix broken buttons after cancelling the modal

Note that with 42184.2.diff, SFTP appears to work as expected. The only issues are with canceling the modal, which happens much more in testing than in practice. [41997] seems like an overreaction to the minor bugs that we need to track down with that, which may seem worse than they are because they are more apparent in a testing workflow than in real usage. Either way, the fixes for that should not be particularly difficult. I'm not planning on working on this issue further, so someone else can take it on.

[westonruter]

See patch https://core.trac.wordpress.org/attachment/ticket/42184/42184.2.diff