Service Desk incoming email should count towards new issue application rate limit

Related to #328238.

!68526 (merged) added rate limiting to all issue creation, keying off project and user.

However, unlike regular incoming email, Service Desk email has no identifiable GitLab user. Instead, the GitLab bot creates the issue on behalf of the user. An exclusion for GitLab bot was made so that this user will not be rate limited.

Rate Limiting should be applied to incoming email for Service Desk by combining project and service_desk_reply_to, instead of project and user. This will ensure that a basic Application Limit applies, even if it's unlikely it would ever be hit.

Update 2021-10-18

Curently service desk issues are not rate limited.

Because RateLimitedService is now used at the Issues::CreateService service level (which is used also when creating service desk issue), we could rate limit also service desk issues (currently issues created by bot are whitelisted).

The following discussion from !68526 (merged) should be addressed:

• @jprovaznik started a discussion:

  Yes, I think this makes sense - I can't think of other option we could do if rate limit is reached.

  But because service desk issues are created under bot user, which is whitelisted, this situation will not occur - we would need rate limit per user's email for service desk incoming issues - is this something to create a follow-up issue?