Improved handling of Markdown in issue descriptions

Recently we noticed that when creating a GitHub issue for issues identified by DeepSource, the GitHub issue description would populate with broken Markdown for certain issues. This was happening because certain issue descriptions, which had code examples and issue details, would get truncated at 50 words.

To fix this we now only populate the Github issue description with the first few lines of the issue description that describe the issue. Code examples & issue details are no longer included in the description.

This fix should remove the need to modify/fix the generated Markdown in GitHub issue descriptions.