David Taylor 830797a9c3
FEATURE: Allow post/topic thumbnails to be prioritized via markdown (#12044)
Previously we would always take the first image in a post to use as the
thumbnail. On media-heavy sites, users may want to manually select a
specific image as the topic thumbnail. This commit allows this to be
done via a `|thumbnail` attribute in markdown.

For example, in this case, bbb would be chosen as the thumbnail:

```
![alttext|100x100](upload://aaa)
![alttext|100x100|thumbnail](upload://bbb)
```
2021-02-11 15:44:41 +00:00
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