Replace user-scoped API tokens with organisation API keys

[matthewelwell]

Currently, when configuring the JIRA integration, Flagsmith users must use their own user token (obtained from their account settings). We'd like to update the integration so that they can use Organisation API Keys instead.

In order to do this, since the prefix in the Authorization header is different depending on the type of token, we'll likely need to consider how we make the change backwards compatible.

Based on the properties of the keys, it looks like we could do something like:

org_api_key_matcher = re.compile("[A-Za-z0-9]{8}\.[A-Za-z0-9]{32}")

if org_api_key_matcher.match(auth_token):
    header_prefix = "Api-Key"
else:
    header_prefix = "Token"