Closes#930.
We already had classes for this kind of feature, but it was used
specifically for repository information. Just renamed the classes so it
is clear how they can be used.
To use them with markdown, just do it like this:
```markdown
{: .only-light}
{: .only-dark}
```
and with html:
```html
<img
class="only-light"
src="assets/img/prof_pic_color.png"
>
<img
class="only-dark"
src="assets/img/prof_pic.jpg"
>
```
Signed-off-by: George Araújo <george.gcac@gmail.com>
summary:
- adds prettier formatter configuration
- formats the entire repo using prettier, ignoring minified files
(`*.min.css`) and heavy generated html
- changes extensions of all `.html` files to `.liquid`, which is more
correct and necessary for prettier to work correctly
- replaces "%-" and "-%" with just "%" — manual liquid formatting using
minus signs is superfluous since we are compressing and minifying the
code anyway
- adds CI action for running prettier check on PR and pushes to master
