Extend plugin

[ioandim]

This pull request expands the notifications for Rollbar webhook events for the following rules:

• High Occurrence Rate
• 10^nth Occurrence
• New Item
• Item Reactivated
• Item Reopened
• Item Resolved
• Occurrence

All of the notifications have a link to the Rollbar item and four more fields. One of these fields is the project title, which is obtained by making a GET call on the Rollbar API.

The link to the Rollbar item has the item’s number, the event type and the item’s name.

High occurrence notifications also report the time of the last occurrence. The timezone for this is included in the response from the call on the Rollbar API.

The changes are the following:

• migrate to the v5 version of the mattermost/mattermost-server/ go modules. Also added new vendor folder. 
• change the makefile and add a build folder similar to the mattermost starter template to support more make commands. 
• Add rollbar api key on the configuration.
• Expand the notifications.