Only syntax highlight if language was specified

[kyrias]

Currently we always try to syntax highlight anything within backticks, which leads to things like fooa_bar being highlighted as autohotkey automatically by highlight.js. There isn't really any good way to autodetect the language for things like that, so instead this PR changes the behavior to only try to syntax highlight if a language was passed when sending a full-fledged fenced codeblock:

```python
import foo
foo.bar
```

This fixes element-hq/element-web#508.

[matrixbot]

Can one of the admins verify this patch?

[kyrias]

Currently foo blocks will not render properly with the dark theme though, because the text color ends up the same color as the background.

[kyrias]

(@ara4n ping? <3)

[ara4n]

sorry for totally missing this - it looks good. Only feedback would be:
a) Can we provide an option to auto-highlight by default for folks who liked the original behaviour (I think this should be trivial via SyncedSettings)?
b) might be worth commenting the two filter() lines just to make it quicker to see what's going on: ("// Filter out all classes other than language-" and "// only run highlight.js on code blocks with explicit language classes" or something)

[kyrias]

Hmm, I'll look into how SyncedSettings works, and how to add a Riot toggle setting then.

[ara4n]

it should be two lines - one to add the definition to UserSettings and one to check it in TextualBody.

[kyrias]

@ara4n Fixed

[ara4n]

lgtm - thanks :)

[turt2live]

:(
This made code blocks on the dark theme illegible.

[kyrias]

cough #1074 (comment) /cough

[kegsay]

The fact that it means code blocks in general are illegible on dark theme is really infuriating, as someone who uses dark theme a lot. I've had to turn on auto-syntax highlighting to fix this. I think we should still default to auto-highlighting, and let people disable it instead.

[kyrias]

Or the style could be fixed instead, and @ara4n said a few weeks ago that he'd done the same fix to the regular theme already, so shouldn't be hard. I have no clue where said stylesheet is, or what he changed though. And the auto-detection is pretty much always wrong, which is why we started looking into disabling it at all.