Backend: Rename CI classes to better reflect CI domain
## Proposal
* Rename `CommitStatus` to `Ci::Job`
* Rename `GenericCommitStatus` to `Ci::StaticJob` ( `Ci::ProxyJob`, `Ci::GenericJob` are alternatives)
- We are calling this `Static` in that it isn't processed by a gitlab runner or triggering a downstream, but these can actually have associated tasks in gitlab such as a deployment of the 'pages' feature. It's more that they are agnostic to what processing is performed and just represent the status of some processing happening somewhere (not on a runner). `Ci::ProxyJob`, `Ci::GenericJob` are alternatives
## Full organization after rename
* `CommitStatus` =\> `Ci::Job`
* `GenericCommitStatus < CommitStatus` =becomes=> `Ci::StaticJob < Ci::Job` (
* `Ci::Processable < CommitStatus` =becomes=> `Ci::ProcessableJob < Ci::Job`
* `Ci::Build < Ci::Processable` =becomes=> `Ci::ExecutableJob < Ci::ProcessableJob`
* `Ci::Bridge < Ci::Processable` =becomes=> `Ci::TriggerJob < Ci::ProcessableJob`
Relations for `Ci::Pipeline`;
* `has_many :jobs` for all
* `has_many :processable_jobs` for `Ci::ExecutableJob` and `Ci::TriggerJob`
epic