mattermod: event handler gets confused between issue comment and PR comment

Description

Found another bug. When an issue is commented on, we try to get PR information assuming that it's a PR. This needs some work around properly parsing the github webhook response, and not just parse the same thing twice and check for non-nil.
```
event := PullRequestEventFromJSON(ioutil.NopCloser(bytes.NewBuffer(buf)))
eventIssueComment := IssueCommentFromJSON(ioutil.NopCloser(bytes.NewBuffer(buf)))
```
The issue comes from here `pr, err := s.getPRFromComment(ctx, *eventIssueComment)`. If this is an issue, then
```
prGitHub, _, err := s.GithubClient.PullRequests.Get(ctx,
*comment.Repository.Owner.Login,
*comment.Repository.Name,
*comment.Issue.Number,
)
```
will fail because comment.Issue.Number is for an issue, but we are querying a PR. Which results in an
```
DEBU[2020-07-09T18:54:11.0501935+05:30] Sending GitHub comment caller="server/github.go:117" comment="Trying to auto merge this PR." issue=14970
ERRO[2020-07-09T18:54:14.0899262+05:30] could not get the latest PR information from github: GET https://api.github.com/repos/mattermost/mattermost-server/pulls/14596: 404 Not Found [] caller="server/server.go:242"
```
Note `GET https://api.github.com/repos/mattermost/mattermost-server/pulls/14596`. 14596 is an issue, but we are querying it as a PR.

Issue created from a message in Mattermost.

QA Test Steps

No testing required.

Mana

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Assignee

Ibrahim Acikgoz

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Reporter

Agniva De Sarker

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Mattermost Team

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