This makes (https) links show up as actual links when they're in a message with a `formatted_body`. For example, the pursuivant messages in any KDE channel (like #kirigami), or links which are in a reply to another message/image.
Also corrected a regex mistake in another place using the same link replacement regex.
Fix#331
Moves the highlight to the timeline container object itself so the whole delegate is highlighted. This was suggested for compact mode but I also left it visible for bubble as I think it still works and adds some user feedback.
Fine tunes the hover actions as follows:
- In compact mode as long as the screen is wide enough they are inset further so they don't overlap the up and down buttons.
- In bubble mode for a user message on the right they are inset slightly further so that they don't overlap the scrollbar
Fine tuned the spacing around the avatar and highlight.
Based upon the suggestions here network/neochat#550
Bubble Before - no highlight can be seen

Bubble After

Compact Before

Compact After

Uses Matrix's preview API to generate embedded link previews.
Only title and description for now.

Replace the custom Placeholder + BusyIndicator with the new LoadingPlaceholder component.
Also remove the "Loading…" page title on first load, as that is redundant to the existing front-and-center "Loading…" message and hence doesn't look nice.
When using Breeze light:
| Before | After |
| ------ | ------ |
|  |  |