Add endpoint to get the latest revision

[hatched]

In the UI for jaas.ai we display a link to the latest revision in the active channel (stable) if you're viewing an old revision. This is determined by us requesting all revisions and then picking the latest. This is a bit overkill and can cause timeouts on charmstore for larger entities.

[mhilton]

@urosj please decide if/when we should do this.

[urosj]

@hatched This does not require a call to revision-info at all. Just make a /meta/id-revision to the charm without specifying the revision in the URL and you'll get the latest revision directly.

Also, the latest revision in the revision-info might not be the latest on the same channel or even the one that is set by the charmer to be used. Hence use /meta/id-revision endpoint