Make Mattermost notifications set up clearer

From a customer: https://gitlab.zendesk.com/agent/tickets/56461

While conducting the “Test Settings” it appears to function but only if you specify the channel name without the # sign which is listed in the placeholder of the textbox, so it was a bit confusing.

Also the test settings appears to only test the first item “Code Push” channel rather than testing all channels.

Here is what I would suggest:

• Remove # from placeholder since this must not be included or it will not function properly

• Test Settings should test all Active / Enabled Notification Channels to ensure a user has configured them all properly.

• Provide Alert Notification Toast messages on failure to user

In my case I had entered “GitLab” into the Mattermost Notifications channel, but realized that while the Channel Name displayed as “GitLab” the actual Mattermost URL endpoint was “gitlab”. I realized my mistake after reviewing GitLab server logs, however, it would be nice to provide context to the user through the user interface for non-system admin users.