Add release event trigger (#3226)

Supersedes #764 

Bitbucket does not support release webhooks.

---------

Co-authored-by: Patrick Schratz <patrick.schratz@gmail.com>

cmd/server/docs/docs.go

@@ -3937,6 +3937,9 @@ const docTemplate = `{
                 "id": {
                     "type": "integer"
                 },
+                "is_prerelease": {
+                    "type": "boolean"
+                },
                 "message": {
                     "type": "string"
                 },
@@ -4383,6 +4386,7 @@ const docTemplate = `{
                 "pull_request",
                 "pull_request_closed",
                 "tag",
+                "release",
                 "deployment",
                 "cron",
                 "manual"
@@ -4392,6 +4396,7 @@ const docTemplate = `{
                 "EventPull",
                 "EventPullClosed",
                 "EventTag",
+                "EventRelease",
                 "EventDeploy",
                 "EventCron",
                 "EventManual"