Application limits for pull mirror API

Users can invoke a pull mirror poll using the API or the repository settings UI.

At present, there appears to be no application limits around these pull mirror invocations.

cc @m_gill @jramsay @timzallmann

Related to gitlab-com/gl-infra/scalability#64

Proposal

We debounce pull mirror invocations to ensure that calls to the API are delayed and batch the pull mirror background jobs up.

As defined in the docs, manual invocation should only be allowed once per 5 minutes.

For example, if we limit projects to 1 pull request invocation per 5 minutes, and the last pull request invocation we 4 minutes ago, we delay the scheduled pull mirror job by 1 minute.