Add request verification to the webhook

Description

Now the service checks the request and its source. If the source of the request is not GitHub or the request does not contain the correct value in the headers by the key X-Hub-Signature-256, then the route of the webhook (POST /) will respond 403 Forbidden.

The validate is performed in accordance with this guide from Github.

What's new in the administration

Previously, the service used a Github token from an GITHUB_TOKEN environment variable. Now the service requires an additional secret token from the environment variables using the GITHUB_SIGN_KEY key. See this guide.

If GITHUB_TOKEN not set, will be raised an exception:

  File "/home/pc/workspace/vk/docbot/docbot/settings.py", line 5, in <module>
    assert token is not None
AssertionError

if GITHUB_SIGN_KEY not set, will be raised an exception:

  File "/home/pc/workspace/vk/docbot/docbot/settings.py", line 6, in <module>
    assert github_signature is not None
AssertionError

Examples

cURL request with correct signature: expected 200;

```bash curl --location --request POST 'http://0.0.0.0:8000/' \ --header 'X-Github-Event: issue_comment' \ --header 'X-GitHub-Delivery: example_delivery' \ --header 'X-Hub-Signature-256: some_correct_value' \ --header 'Content-Type: application/json' \ --data-raw '{ "issue": { "state": "open", "url": "http://0.0.0.0:8000", <-- change it "html_url": "http://0.0.0.0:8000" <-- change it }, "repository": {"test" :"bugbounty", "full_name": "test"}, "action": "created", "comment": { "body": "@TarantoolBotdocument\r\nTitle:fdsf", "user": { "login" : "test"} } }' ```

cURL request without signature: expected 403;

```bash curl --location --request POST 'http://0.0.0.0:8000/' \ --header 'X-Github-Event: issue_comment' \ --header 'X-GitHub-Delivery: example_delivery' \ --header 'Content-Type: application/json' \ --data-raw '{ "issue": { "state": "open", "url": "http://0.0.0.0:8000", <-- change it "html_url": "http://0.0.0.0:8000" <-- change it }, "repository": {"test" :"bugbounty", "full_name": "test"}, "action": "created", "comment": { "body": "@TarantoolBotdocument\r\nTitle:fdsf", "user": { "login" : "test"} } }' ```

cURL request with bad signature: expected 403;

```bash curl --location --request POST 'http://0.0.0.0:8000/' \ --header 'X-Github-Event: issue_comment' \ --header 'X-GitHub-Delivery: example_delivery' \ --header 'X-Hub-Signature-256: haha' \ --header 'Content-Type: application/json' \ --data-raw '{ "issue": { "state": "open", "url": "http://0.0.0.0:8000", <-- change it "html_url": "http://0.0.0.0:8000" <-- change it }, "repository": {"test" :"bugbounty", "full_name": "test"}, "action": "created", "comment": { "body": "@TarantoolBotdocument\r\nTitle:fdsf", "user": { "login" : "test"} } }' ```

Closes tarantool/security#124

tarantool / docbot