Fixes #23860.

My initial idea was to finally remove all the local icons and fetch them remotely, but it's not currently possible because half of these remote icons are outdated and some of the (Threads) have no background and don't work in dark mode. So the app has to have local overrides.

Changes:

• Update JetpackSocialNoConnectionView.swift to cache remote icons using CachedImageView
• Add new override for Threads (fixes dark mode)
• Add support for fetching remote images in SharingViewController.m (site Sharing Settings).

Unfortunately, there is still one more place that needs to be updated and it's not perfect, but I ran out of time, so maybe next time. It fixed the most obvious issues.

Before

After

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)