Code colors (backend support)

[paskal]

Backend part for #32.

In order to merge it, we need clearance from frontend developers that getting rid of <code> tag won't break anything and they will be able to add CSS to render this code properly.

Also, all review comments should be resolved before the merge.

Example without the language specification

Comment text

```
test text
```

Old render

<pre><code>test text
</code></pre>

New render

<pre class="chroma">test text
</pre>

Example with the language specification

Comment text

```go
func main(aa string) int {return 0}
```

Old render

<pre><code class="language-go">func main(aa string) int {return 0}
</code></pre>

New render

<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">(</span><span class="nx">aa</span> <span class="kt">string</span><span class="p">)</span> <span class="kt">int</span> <span class="p">{</span><span class="k">return</span> <span class="mi">0</span><span class="p">}</span>
</pre>

[github-actions]

Pull Request Test Coverage Report for Build ddc5f119d-PR-590

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 83.344%

---

Files with Coverage Reduction	New Missed Lines	%
backend/app/cmd/server.go	2	80.86%

Totals
Change from base Build b566d7f84:	-0.02%
Covered Lines:	5074
Relevant Lines:	6088

---

💛 - Coveralls

[akellbl4]

Tested with styles #592
LGTM

[umputun]

LGTM