Support for more than the opened GitHub action in the _branch_request file

[dmarcoux]

Right now, the method check_for_branch_request only supports the opened GitHub action. Since we don't do anything with the value of the GitHub action, it could be anything and it would still be fine. My proposal is to only check if the action key is present the _branch_request file, not for its value. This way, we can still tell apart GitHub and GitLab based on the presence or not of the action and object_kind in the _branch_request file.

[hennevogel]

My suggestion would be to not rely on the payload from github/gitlab at all for this inside the service. We abstract this in the frontend or other clients. So _branch_request can just be some simple json file like

url: https://some.url.some.where
revision: df957ace574

or for the sake of consistency I would say it should be XML

<branch_request>
  <url>https://some.url.some.where</url>
  <revision>df957ace574</revision>
</branch_request>