Allow multiple Slack endpoints with finer-grained conditions

[benmandrew]

Intended to allow failures of the opam2web pipeline to be pinged to the #opam Slack channel, while preserving the behaviour of pinging all status updates of all repositories to #ci-firehose.

---

Changes the input slack file to be in JSON, the format of which is a list of objects with fields:

• uri, the URI endpoint for the Slack application, of the form "https://hooks.slack.com/services/***/***/***"
• mode, which is the condition on which we send a Slack message. Either "all" (send on all status updates) or "failure" (only send on deploy failures).
• repositories, an optional parameter. If it is not present, then we apply the record to all repositories being deployed. If it is present, then it must contain a list of repositories, each being represented as a string of the format "org/repo", e.g. "ocurrent/ocaml-ci".

Here is an example of a valid JSON string:

[
  {
    uri:"https://hooks.slack.com/services/***/***/***",
    mode:"failure",
    repositories:["ocurrent/ocaml-ci", "ocurrent/opam-repo-ci"]
  },
  {
    uri:"https://hooks.slack.com/services/***/***/***",
    mode:"all"
  }
]

The first record in the list says that when there is a deploy failure on the ocurrent/ocaml-ci or the ocurrent/opam-repo-ci repos, send a Slack message to the specified URI.

The second record says that for every repo, for each event on the deployment of those repos (successes and failures), send a Slack message to the specified URI.