Format code fence into jira code block

[timja]

In both issue creation and in comments we could benefit from convert code fenced blocks to jira code markup

```
hello
```

to
{code}
{code}

bonus points for language support:

```yaml
my_yaml:
blah: true
```

to:

{code:yaml}
my_yaml:
blah: true
{code}

check all examples (doing from memory)