Allow Bitbucket users to create personal subscriptions for Slack to repositories they have read/write access

Problem Definition

Bitbucket users can subscribe Slack channels to Bitbucket repositories only if they have administrator access to these repositories.

This means that users with read/write access to a repo cannot create a personal subscription to such a repo.

Suggested Solution

Allow users with read/write access to a repo to create a personal subscription to that repo.