Add option to send without channel defined

[Ataraxxia]

In the most recent version of mattermost + Graylog 3 I am not able to send notification, not even test. I have pinned down the problem down to getting 404 on my Nginx reverse proxy pointing to mattermost server. I need to mention that I have other incoming webhooks working just fine.

In the current version of mattermost, webhook can be defined with predetermined and locked channel, and I have wintessed similar behaviour of getting 404's when having channel name defined. Simple option to omit channel name in POST request would suffice I think.

83.12.190.58 - - [13/May/2019:18:42:10 +0200] "POST /hooks/HOOK_ID HTTP/1.1" 404 163 "-" "Java/1.8.0_212"
83.12.190.58 - - [13/May/2019:18:43:39 +0200] "POST /hooks/HOOK_ID HTTP/1.1" 404 163 "-" "Java/1.8.0_212"
83.12.190.58 - - [13/May/2019:18:49:45 +0200] "POST /hooks/HOOK_ID HTTP/1.1" 404 163 "-" "Java/1.8.0_212"

[Ataraxxia]

Proposed a pull request:

#86

[pndvg]

Hi all!
I found the solution in: https://developers.mattermost.com/integrate/webhooks/incoming/?utm_source=mattermost&utm_medium=in-product&utm_content=confirm_integration&uid=9a444bijbjdtxx4tjzi1ozbo1e&sid=49oosgy1kfdxbbimm8dsd4muro
Just read carefully the part about "channel": names of the channels a slightly changed, when you send notifications to them from graylog. Use lover case letters, replace spaces with "-". For example original name of channel "95 Tests" converts into "95-tests" in send message notifications metadata.