Add syntax highlighting to the source string

[alvinhochun]

Is your feature request related to a problem? If so, please describe.

For some message types (RST for example), Weblate adds syntax highlighting to the translated string, but the source string remains un-highlighted. This confuses some translators initially.

If both source string and translated string have consistent highlighting, it will also make it easier to match the markup between source and translation.

Describe the solution you'd like

Add the same syntax highlighting to the source string display.

Describe alternatives you've considered

Screenshots