feat: show galleries as sliders

[DokterKaj]

Todo:

• Show galleries in posts as sliders
• Add an indicator that a post is a gallery (+ display number of images)
• Show galleries in post lists as sliders
• Add settings option to toggle between showing galleries as sliders or unrolled

Possible additional ideas:

• Implement vertical sliding direction
• Implement dot indicators using scroll-timeline

[paidlowered]

This is an awesome idea!