Agenda item number should get updated if the items of the agenda are deleted

[marianunez]

The agenda items' numbering gets off by one when the first item is deleted.
The numbering is also off when the items are manually edited but might be difficult to tackle as Maria Nuñez has mentioned here

QA Test Steps

1. Ensure that the agenda plugin is installed and enabled.
2. Go to a channel header and edit the agenda settings to update the hashtag if needed (e.g.: test-Jan02).
3. Use command /agenda queue to queue at least 3 items as following:
   /agenda queue item 1
/agenda queue item 2
/agenda queue item 3
4. Click on the agenda hashtag to open up the agenda list on the RHS.
5. On the RHS, click the post menu for item 1 (or the first item that was queued) and delete the agenda item.
6. Ensure that now there are only 2 items for that agenda hashtag: item 2 and item 3 on the RHS.
7. Add a new agenda item: /agenda queue new item.
8. Click on the hashtag to refresh the RHS.

Observed: The RHS has 3 agenda items numbered 2), 3) and 3)

Expected: The RHS has 3 agenda items numbered 1), 2) and 3)

Jira: https://mattermost.atlassian.net/browse/MM-26536