Improve Link Preview Sizing

Update linkpreviewer to minimise the number of resizes.

• There is now a placeholder loading element to hold space while the data is obtained
• The component starts with a set size but can be expanded to see the whole item.
• The expand/contract button is only visible on hover
• The default size is picked to show 3 lines of text (1 heading and 2 description, 2 heading and 1 description, etc)

This is designed to help with the timeline scroll jumping CCBUG: 463235 by minimizing the amount of resizing for items in the timeline.

Before

After when compressed

After When expanded

After Loading